5 %-^ 



Iron Tannage 



DISSERTATION 

SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR 

THE DEGREE OF DOCTOR OF PHILOSOPHY IN THE 

FACULTY OF PURE SCIENCE. COLUMBIA 

UNIVERSITY IN THE CITY OF 

NEW YORK 



BY 
Te.Pang Hou, S. B., M. A. 



NEW YORK. U. S. A. 
1921 



4- 



r5i ^7 

.Hi. 







P4 



^ 



To Allen Rogers to whom the author owes his first interest 
in tanning this work is dedicated. 



SPECIAL ACKNOWLEDGMENTS. 

The author desires to express his indebtedness to Professor 
Daniel Dana Jackson, head of the Chemical Engineering Depart- 
ment, Columbia University, without whose guidance and gen- 
erous help this work could not have been successfully carried 
out. 

To Dr. Allen Rogers, Pratt Institute, Brooklyn, N. Y., he 
wishes to express his thanks for the helpful suggestions and for 
the use of the tannery under his charge for fully one-half of a 
year. 



CONTENTS 



Section I. 
Section II. 
Section III. 

Section IV. 

Section V. 

Section VI. 
Section VII. 

Section VIII. 
Section IX. 
Section X. 
Section XI. 
Section XII. 
Bibliography 
Appendix 



General Discussion 

History of Iron Tannage Including Recent Work 

Investigation of Different Methods of Oxidation 
and Some Methods of Preparation of Iron 
Liquor from Copperas 

Hydrolysis and Decomposition of Ferric and 
Chromic Salts Compared 

On the Relation of Basicitj' to Stability in Iron 
Liquor 



PAGE 
I 



14 



28 



35 



Behavior of the Pelt towards Iron Tan Liquor 43 



Experiments on Tanning with Ferric Hydroxide 
Hydrosol 

General Experimental Work on Iron Tanning 

Chrome Iron Joint Tannage 

Pure Iron Tannage 

Iron Phosphate Tannage 

Conclusions 



SI 

54 
59 
64 
69 
72 



The More Important Works in Leather Industry 74 

A Tentative Procedure for the Ordinary Chemical 

Analysis of Iron-Tanned Leather tj 



[Reprinted from Journal of American Leather Chemists 
Association, February, March, April. May, 1921.] 

IRON TANNAGE. 

Section I. General, Discussion. 

As early as the latter half of the eighteenth century iron salts 
as tanning agents were proposed and experimented upon. From 
that time on attempt after attempt was made to obtain a satis- 
factory tannage with iron, but without material results. Differ- 
ent experimenters took up the matter with renewed enthusiasm, 
each cherishing a hope of thereby revolutionizing the tanning in- 
dustry, but plants running on such processes were unsuccessful. 
When it is considered that next to alum, iron compounds were 
among the first inorganic salts introduced for mineral tannage, it 
may be realized how far iron tannage has fallen behind that of 
chrome and even of alum tannage at the present time. Diversified 
reasons have been given by different investigators to account for 
the unsatisfactory products obtained. No doubt, while some of 
these represent true causes responsible for the failure, much is 
due to a lack of understanding of the peculiar properties of the 
iron salts rather than to the intrinsic character of iron. It has 
been our object to make a study of iron tannage and to determine 
wherein the difficulties lie and how they may be met. 

True to the general trend of the Periodic Table, aluminum with 
the lowest atomic weight is the most acidic, while iron having the 
highest atomic weight is the most basic, with chromium lying in 
between.^ This accounts for some of the differences in the be- 
havior of the three elements as tanning agents. But there seem 
to be, as far as the tanning properties are concerned, more funda- 
mental differences, not in degree but in kind, which should be 
ascribed to their distinct properties as individual elements. For 
instance, both iron and alum tanned leathers do not resist boiling 
temperature, whereas chrome tanned leather is not affected unless 
it is subjected to boiling continuously for a considerable length of 
time. 

Some investigators seem to have worked along the idea that 
the basic ferric sulphate corresponding to the formula Fe(OH)- 
SO4 is the compound in the liquor that brings about tanning, and 

^ See Stieglitz, "Quantitative Chemical Analysis," Vol. I, p. 195 (1919). 



2 I^EATHER CHEMISTS ASSOCIATION 

have striven to produce such a basic ferric saU liquor for this 
purpose. This is too basic for a sulphate liquor, although ferric 
chloride liquor can stand a higher basicity than this. Ferric sul- 
phate in solution corresponding to this degree of basicity is not 
stable and the hyd.rated ferric oxide, FcgOs-xHaO, a fine yellow 
crystalline precipitate, will invariably separate out within a short 
time, even when the liquor is not diluted. If the liquor is diluted, 
turbidity is almost instantaneous with a rapid separation of the 
light yellow precipitate. To produce a good tannage with iron, 
the basicity of the liquor employed is found to be considerably 
less than this in the case of the sulphate and the range between 
which the basicity may vary is rather narrow. Symbolically the 
salt in the ferric sulphate tan liquor may be represented by Feg- 
(0H)jn.(S04)^ where x has a value between i and i}4 and y 
between 2^ and 2j4, but there is no definite formula assignable, 
as there is no sharply defined chemical compound and, besides, 
when it comes to neutralization, the iron that is fixed in the pelt 
is of a highly basic character. But any attempt to bring about 
the formation of such a basic salt as is represented by Fe(OH)- 
(SO4) invariably causes a precipitate of FcgOs-xHaO. This 
FegOg.xHgO is entirely inert and we can not expect to obtain a 
tannage from it any more than from a precipitated CraOg.xHaO if 
such is produced in the chrome liquor from the use of too much 
alkali. The oft reported "hard and brittle leather" may be simply 
due to improper tannage from the presence of much precipitated 
FcgOa-xHaO. The difficulty with the iron liquor, then, is that the 
ferric salt, particularly the sulphate, is very readily hydrolyzed 
and that, what is still worse, upon hydrolysis the ferric hydroxide 
seems to pass through the colloidal range so rapidly that it is 
soon flocculated or coagulated as a light yellow crystalline sub- 
stance FegOg.xHaQ. This will be appreciated when it is seen that 
Al(OH)3 upon hydrolysis remains in the colloidal realm for a 
considerable time, and when finally separated out from the solu- 
tion it does not readily become dehydrated as AI2O3.XH2O. The 
same is true of Cr(OH)3 from a chromic salt solution, and even 
more pronounced. 

Another difficulty is that ferrous iron in solution, though easily 
partially oxidized is rather difficult to be completely oxidized. To 
effect a partial oxidation is one thing, but to oxidize completely all 



IRON tannage; 3 

the ferrous iron in solution to the ferric state seems quite another. 
Oxygen from the air will partially oxidize some ferrous iron in 
solution to the ferric state, but it never is able to oxidize it com- 
pletely, except in alkaline solution. Perchloric acid, another ox- 
idizing agent, oxidizes iron in solution partially but complete oxi- 
dation is quite difficult. With Ig, complete oxidation is impossi- 
ble, although a small amount of Felg is obtained. There is nor- 
mally always an equilibrium between the ferrous iron and the 
ferric iron in solution. To shift the equilibrium to completion 
requires a much higher potential which is only secured by using 
a powerful oxidizing agent, or for the same oxidizing agent a 
higher concentration of this. The equilibrium ratio is 

(Fei'i) 

Thus, a small concentration of ferric iron present in solution is 
normally sufficient to set up an equilibrium and it takes a con- 
siderable potential to shift the equilibrium to the ferric end. 
This consequently calls for the presence of some excess of a 
powerful oxidizing agent to prevent any of the ferric iron from 
being again reduced. Complexities result from the fact that iron 
is capable of existing in two different states, of which the ferric 
state is the one that possesses the tanning property and this gives 
rise to one of the great difficulties in iron tannage. 

In the case of aluminum the problem is simpler, as aluminum 
does not exist in a state other than the trivalent, and it has little 
or no tendency to be flocculated into a similarly dehydrated 
AI2O3.XH2O compound in the solution. 

As to chromium the condition is, on the other hand, decidedly 
favorable. For, while chromium does exist in other states, notably 
as Cr04=, or Cr207=, it would take an excessively high oxidation 
potential to cause it to go to the chromate state, except in an 
alkaline solution, which is not the case with the chrome bath. 
Hence to all intents and purposes chromium under such condi- 
tions can be said to possess only one state of oxidation, i. e., Cr"^ 
as Is also the case with Al. Further, tanning conditions are far 
more favorable to reduction than to oxidation. The protein 
' See Stieglitz, "Quantitative Chemical Analysis," Vol. I, pp. 270-275 
(1919)- 



4 i,e;athe;r chemists association 

bodies in the hides and skins, the woody material of the drum or 
paddle, and the metallic joints of the apparatus all possess dis- 
tinct reducing properties. Ferric salt in the liquor which is so 
susceptible of reduction is always reduced to a greater or less de- 
gree, as can be easily proved by testing the spent liquor with potas- 
sium ferricyanide solution after the tanning operation. In this 
connection it can not be too strongly emphasized that the tan liquor 
should not be left in contact with a wooden or iron container un- 
necessarily, and should not be introduced into the drum until the 
tanning operation is ready to begin,. Much trouble has been traced 
to the iron liquor being materially reduced and converted to the 
ferrous state during the progress of tanning. For chrome tan- 
nage, this condition is exactly what we desire as chromium is to 
be kept at its lower state of oxidation, namely the chromic state. 

From the above it is clear why there are added difficulties in 
the case of iron tannage. It is necessary, first of all, to convert 
the ferrous iron to the ferric state completely, then to keep it in 
this ferric state throughout the tanning operation under adverse 
conditions, and to prevent the separation of any hydrated ferric 
oxide, Fe^Os.xHgO, by adjusting the proper acidity in the liquor 
employed. 



IRON tannage: 5 

Section II. History of Iron I^annage, Including 
Reicent Work. 

Attempts to use iron salts as tanning agents date back to the 
time when efforts were first made to find a substitute in the form 
of metallic salts for vegetable tannins. In the course of more 
than a century, efforts were repeatedly made and interest was 
continually revived to make iron tannage a commercial success, 
but without reward, in spite of the great promise that inspired 
such investigators to make a most determined effort. Within the 
last decade, partly on account of the Great War, new interest has 
been given to iron tannage, and the properties and behavior of 
iron salts as tanning agents are gradually becoming better under- 
stood. 

The history of iron tannage begins from the latter part of the 
eighteenth century. Many of these early investigators are men 
who also helped to establish the present-day chrome tannage. 

In the year 1770, J. Johnson,^ an Englishman, patented a pro- 
cess of tanning using ferrous sulphate with an acid (sulphuric 
acid, hydrochloric acid or nitric acid). The pelt was tanned in 
three operations in the middle of which a vegetable tannin was 
used. 

In 1794, Sam. Ashton,* another Englishman, recommended the 
use of a mixture of iron oxide and sulphuric acid, calcined iron 
ore or iron ochre with pyrites, copper ore, and zinc for tanning. 
The duration of tanning was given as from five to seven weeks. 
For calfskins some alumina was also added. 

In 1805, Sigmond Hermbstadt^ in his book on leather tanning 
explained that solutions of metallic salt baths had similar action 
on the pelt as the oak tannins. Among other salts he mentioned 
the red iron sulphate, in which not only the grain of the pelt was 
affected, but the pelt was virtually converted into leather if soaked 
in it for some time. He prepared his iron tan liquor by heating 
ferrous sulphate to a dry yellow substance which was dissolved 

^ Handbuch der Chromgerhung, Josef Jettmar, p. 133 (1900); also 
Die Chromgerhung, J. Borgmann, p. 6 (1902). 
*Ihid. 
^ "Le Tannage au Fer," Le Cuir, Josef Jettmar, June 15, 1919 



b i,e;athe;r chemists association 

in twenty times its volume of boiling water, and on cooling, the 
clear yellow-red solution decanted for use. He also treated iron 
oxide with acetic acid, or oxidized ferrous sulphate with concen- 
trated nitric acid or with a mixture of concentrated nitric acid 
and sulphuric acid. Sole leather as well as upper leather could 
be made in this way but the leather obtained was detanned in con- 
tact with water. 

In 1842, d'Arcet,^ a Frenchman, tanned the hides in a ferric 
sulphate solution but the sulphuric acid set free gradually de- 
stroyed the hides. In the same year, Julius Bordier,'^ of London, 
patented a process (British Patent 9,219, 1842) of oxidizing fer- 
rous sulphate with nitric acid and sulphuric acid, and with man- 
ganese dioxide and sulphuric acid. It was said that he had at- 
tained some success. 

In 1853, Hylten Cavalin,^ employed for tanning, liquor contain- 
ing 10 pounds of dichromate of potash and 20 pounds of alum in 
180 pounds of water. The hides were tanned in this liquor for 
four days, and were next placed in a 10 per cent, ferrous sulphate 
solution for twelve hours with frequent stirring. The acidity re- 
lation of these two liquors was not properly adjusted and the iron 
was not completely oxidized. The leather obtained was hard and 
brittle. 

In 1855, Rene de Kercado Molac and Jean Daniel Friedel,» both 
of Strasbourg, France, patented a process whereby the hides 
were tanned in a basic ferric sulphate solution which was later 
neutralized with metallic oxides, such as ferric oxide, alumina, 
and zinc oxide to remove the sulphuric acid liberated. They pre- 
pared the liquor with ferrous sulphate, manganese dioxide and 
sulphuric acid, and added to the liquor ferric acetate in varying 
proportions. A. E. L. Bellford, of London, patented their pro- 
cess in England. In this British patent (January 12, 1855) it 
was stated that the leather treated by other mineral processes is 
"liable to tear in length of time on account of the great quantity 
of acid remaining in the leather corroding the animal fibres." 

""Leather Industry," A. M. Villon, trans, by F. T. Addyman, p. 189 
(1901). 

' See also "The Arts of Tanning," Campbell Morfit, p. 396 (1852). 
'"The Arts of Tanning," Campbell Morfit, pp. 397-401 (1852). 
* Compare "Handbuch der Chromgerbung," Jettmar, p. 137. 



IRON TANNAGE 7 

Dr. Frederich L,. Knapp, Professor of the Polytechnic School 
of Brunswick, Germany, made a thorough, scientific investigation 
on these mineral tannages and published the results in "Die Natur 
und das Wesen der Gerberei und des Leders" (Munich, 1858), 
and also in an article, "tJber Gerberei und Leder," in Dingler's 
Polytechnische Journal, Vol. 181, p. 311 (1866). He made a satis- 
factory explanation of the tanning action. He had in mind the 
possibility of reducing the length of time needed in the vegetable 
tanning process and eliminating the costly materials such as egg- 
yolk and flour used in alum tannage. He recognized the plump- 
ing effect upon the hides by the acid liberated during tanning and 
the stiff and brittle character of the leather obtained. He recom- 
mended neutralizing the tan liquor during the progress of tanning 
with sodium carbonate or caustic soda and pointed out the advan- 
tages in so doing, namely, that the hides were more richly tanned, 
that the harmful acid effect was prevented, and that a neutral 
electrolyte NaCl (in FeClg liquor) was produced in the tan liquor. 
His English patent (British Patent 2,716, 1861, through John H. 
Johnson) covered iron, chrome, manganese, and other metallic 
salts in combination with fatty acids to form insoluble metallic 
soaps so that the iron in the pelt might not be washed out. He 
also mentioned the use. of similarly insoluble silicates of alumi- 
num and alkaline earths. According to his patent (German Pat- 
ent No. 444, 1877) he prepared his liquor by adding nitric acid in 
excess to oxidize completely a boiling ferrous sulphate solution 
when brown nitrogen dioxide (NO2) fumes were seen. After all 
iron was oxidized he introduced more ferrous sulphate into the 
resulting solution as long as NO2 fumes were evolved. The liquor 
after evaporation becomes a varnish-like liquid. Judging from 
this description his liquor must have been too alkaline through 
the loss of the nitric acid by boiling. In his additional patent 
(German patent No. 10,518, 1879) he used instead of the nitric 
acid, sodium nitrate and sulphuric acid for oxidation. This 
method is far more economical and involves no danger of losing 
the acid by heating so that the acidity of the resulting liquor is 
under control. Furthermore, a neutral salt, Na^SO^, is produced 
in the tan liquor. 

In 1864, F- Pfannhauser^*' obtained a patent for the preparation 

""Manufacture of Leather," Chas. T. Davis, p. 290 (1897). 



8 i^^ATHER che;mists association 

of a basic ferric sulphate solution and its use in tanning. He 
roasted ferric sulphate to a red heat with continuous stirring until 
it was reduced to a red powder which was then thrown into water 
while still hot. Most of this powder was said to be dissolved. 
The suspension was allowed to settle and the supernatant liquid 
drawn off for the preparation of tan liquors of varying strength. 
The skins were tanned countercurrently and, when tanned, placed 
in a soap solution. 

In 1877 Paesi^^ proposed to use a ferric chloride solution to- 
gether with salt at 20° C. in the ratio of 100 parts of water to 
ten parts of FeClg and five parts of salt. 

In 1 88 1, E. Harcke obtained a German patent, No. 19,633, 
according to which the pelt for making sole leather was treated 
with a mixture of a resinous body (such as rosin), coal tar creo- 
sote, or carbolic acid, and an alkali, in water, until thoroughly 
penetrated. The pelt was then tanned, first in an aluminum salt 
solution and then in a ferric chloride solution, or other ferric salt 
solution. For making upper leather the hides were previously 
limed and if softness and porosity were desired, the rosin could 
be omitted. 

In 1881, W. Eitner^^ patented a process (Austrian Patent No. 
6,775 ) using a mixture of a basic chromic sulphate and ferric 
sulphate solution. This process was used in Graz, Austria, and 
the product known as "Patentleder, Marke Elefant." By chang- 
ing the ratio of the chromic salt to the ferric salt different grada- 
tions of color — from yellow (of the iron) to gray (of the mix- 
ture) and to green (of the pure chrome) — were obtained. When 
a mixture of the ferric and chromic salts was used, the leather 
was colored black with logwood alone; when chromic salt alone 
was used, the leather was colored black with logwood and an 
iron "striker." When yellow color was not desired in the pro- 
duct, chromic salt alone was used for tanning. Leather obtained 
in this way was stuffed, after sammying, with mixtures of train 
oil, castor oil, stearin, tallow, mineral oil, etc., with soda bicar- 
bonate, soap, borax, casein, etc., as emulsifying agents. 

""Leather Industry," A. M. Villon, trans, by Addyman, p. 189 (1901). 
^"Die Chromgerbung," J. Borgmann, pp. 49-54 (1902); also "Hand- 
buch der Chromgerbung," Jettmar, p. 151 (1900). 



IRON tannage; 9 

In 1886, John W. Fries, of Salem, North Carolina, patented a 
process of tanning (U. S. Patents Nos. 343,166 and 343,167) 
using ferrous carbonate (or ferrous sulphate), sodium carbonate 
(or sodium bicarbonate) and sulphuric acid. The skins were 
tanned first in a dilute liquor for two or three days and then in 
a more concentrated liquor for the same length of time. A small 
amount of sugar might be added. After the tanning operation 
the skins were hung in the air to get the iron oxidized. For 
currying, he used tallow with a paraffin oil, lard, or cotton seed 
oil, and later, in his patent No. 343,167, he recommended an alco- 
holic solution of castor oil. 

In 1892, Paul F. Reinsch, Erlangen, Bavaria, patented a pro- 
cess (German Patent No. 70,226) using a liquor prepared by mix- 
ing 10 kg. FeClg dissolved in 40 1. of water with 4^ kg. crystalline 
NagCOa dissolved in 20 1. of water, thus yielding a dark brown 
solution. He called it ferric chloride-sodium chloride liquor, 
which he used for making different kinds of leather either alone 
or in combination with alum-sodium chloride tannage. In 1912 
he obtained another German patent. No. 265,914, on the use of 
ferric chloride and magnesium carbonate. He prepared the liquor 
by dissolving i kg. ferric chloride in 4 1. of water to which was 
added a suspension of 225 g. MgCOg in a liter of water. To this 
mixture he added a solution of 8 per cent, aluminum chloride. 
Evidently his idea is to bring about the required basicity by 
MgCOg. The AICI3 present is probably meant to help keep the 
basic ferric chloride in solution. 

J. Bystron and Karl Baron von Vietinghoff obtained a number 
of German patents, Nos. 255,320, et seq., in 191 1, a British patent. 
No. 13,952 in 1912, and two U. S. patents. No. 1,048,294 in 1912 
and No. 1,061,597 in 1913. They employ nitrogen dioxide, NO,, 
and nitrogen trioxide, N2O3, for the oxidation of iron. The nitric 
oxide, NO, from the oxidation reaction is collected and reoxidized 
by contact with fresh air to NO2 and N2O3, which gases are used 
over again for oxidation. They thus proposed to utilize the NO2 — 
NO — NO2 cycle, making the oxides of nitrogen virtually catalytic 
agents for the oxidation of iron. In the British patent. No. 13,952, 
they observed considerable precipitates formed in the tan liquor 
and on the skin. According to them, the presence of large quan- 



lo i,e;athe;r chemists association 

titles of an acid causes the formation of a highly acid and not 
completely insoluble iron oxide in the skin so that the leather 
made is brittle and can not be stored. In this patent and also in 
the U. S. patent No. 1,048,294 he proposed placing the skin in a 
ferrous salt solution and oxidizing the ferrous iron by passing in 
NO2 gas from outside or by liberating HNO2 from a nitrite added 
to the liquor. Thus they attempt to combine the oxidation reac- 
tion and the tanning operation in a single procedure. It is true 
that HNO2 (from a nitrite and an acid) has sufficiently high oxi- 
dation potential to oxidize ferrous iron to the ferric state, but in 
order to oxidize all the ferrous iron into the ferric state com- 
pletely, the presence of much acid in the solution and of an excess 
of the oxidizing agent is needed. If the oxidation by HNO2 or 
oxides of nitrogen is to take place simultaneously with tanning 
operation at the low acidity necessarily present in the tan liquor, 
probably there will be much difficulty in getting all of the ferrous 
iron completely oxidized. Bystron in the U. S. patent No. 1,061,- 
597 patented the use of a neutral alkali salt such as Na2S04 or 
NaCl for treating the iron-tanned leather. He claimed that by 
this treatment a more insoluble basic ferric salt of a light color 
is formed in the leather, thereby yielding a soft, elastic, and non- 
brittle leather. 

O. Rohm in 1917 obtained British patents Nos. 103,827 and 
104,338 on the combination tannage using formaldehyde and 
ferric chloride, or formaldehyde and a mixture of ferric chloride 
and chromic chloride or aluminum chloride. In his patent No. 
103,295 (not accepted) he mentioned the use of ferric alum mixed 
with vegetable tannins to form iron tannate (ink) for tanning. 
In his patent No. 103,827 he recommended tanning with formal- 
dehyde in sodium bicarbonate solution followed by a tannage with 
a ferric chloride solution, a mixture of ferric chloride and chromic 
chloride, a mixture of ferric chloride and aluminum chloride, a 
ferric chloride solution and then vegetable tannins, or a ferric 
chloride solution with an alkaline sulphide. He also mentioned 
the treatment of the skin with an iron precipitant, such as NH3, 
alkalies, or alkaline salts; or phenols, naphthols, organic carbox- 
ylic acids, vegetable tannins ; or soap, sulphide, polysulphide, and 
the like. He mentioned that the leather obtained would not be- 
come slippery in wet condition as is the case with a chrome 



IRON TANNAGE II 

leather. In his patent No. 104,338 he stated that the aldehyde 
tannage could be advantageously used to follow iron tannage after 
neutralization or together with neutralization. When the alde- 
hyde is introduced together with the neutralization after the iron 
tannage, there is, according to his observation, an advantage that 
the grain-drawing so common in a mineral tannage will be pre- 
vented. His thought seems to be along the line that since alde- 
hyde tannage is carried on in an alkaline solution, the introduc- 
tion of the aldehyde tannage after the iron will serve also as a 
neutralization operation to fix the iron in the pelt. We have tested 
this combination tannage and found the leather so obtained satis- 
factory. But since formaldehyde is a tanning agent by itself, to 
what extent the iron salt has contributed to the tannage is diffi- 
cult to tell. 

Emil Kanet^^ in his German patent No. 306,015 (1918) intro- 
duced an interesting feature in the mode of tannage. He derived 
the tanning action by the hydrolysis of a ferric salt. He treated 
the pelt at a low temperature with a ferric salt solution of such 
a basicity that it would be unstable at the ordinary temperature, 
and, after allowing the liquor to penetrate the pelt, raised the tem- 
perature to bring about hydrolysis. To illustrate, he placed the 
skins in a basic ferric acetate liquor containing from y^ to 2^ 
per cent. V^^Q^, preferably with the addition of some salt or other 
electrolyte such as sodium acetate. After the skins were pene- 
trated by the tanning liquor he transferred them to a fairly con- 
centrated salt solution at a temperature of from 45° to 60° C, or 
exposed them to heat in a warm chamber. The tanning action 
was completed in a short time but the stock was further laid 
aside for some time to fix the iron. The acetic acid set free 
under the influence of heat can be recovered from the skins by 
pressure. If a filling material such as flour is used with the tan 
liquor, it is, according to him, fixed in the leather with the basic 
ferric acetate. Other mineral salts such as chromic salt can be 
mixed with the iron. The advantage claimed is that at a low 
temperature a more basic ferric salt solution can be used and that 
the oxidizing activity of the ferric iron towards the skins is 
lessened. 

^^ Compare also "Le Tannage au Fer," by J. Jettmar, Le Cuir, July i, 
1919- 



12 IvEATHER CHEMISTS ASSOCIATION 

W. Mensing in his Swiss patent No. 75,775 in 1918, recognized 
the ease with which ferric salt in solution is decomposed and 
mentioned the effect of ferrous iron upon the skin when the fer- 
rous salt is present in the tan liquor. He recommended the use 
of an excess of an oxidizing agent and patented the use of a 
chlorate (Na, K, or Ba) as the oxidizing agent. He also recom- 
mended a preliminary treatment of the skin with borax or a basic 
aluminum or chromic salt solution for the use of a slightly more 
acid or neutral ferric liquor. According to his idea the tanned 
stock should not be washed with water but only wrung or pressed 
to get rid of the excess of the tan liquor. On drying, the stock 
is oiled with a mineral oil, paraffin or ceresin and then washed. 
To avoid reaction of the iron in the pelt with vegetable tannins 
he recommended fixing the iron by treating the leather with a 
slightly alkaline solution before vegetable retanning. He advo- 
cated the bleaching of the leather by detanning the surface layers 
by means of a reducing agent and then an acid. On the whole, 
his patent marks a better understanding of the properties of the 
iron tan liquor and the process of iron tannage. 

Vittorio Casaburi,^* in the articles, "Notes on the Tannage of 
Skins with Iron Salts," published the results from a series of his 
experiments, using a basic ferric sulphate solution ( from the oxi- 
dation of ferrous sulphate with a mixture of nitric acid and sul- 
phuric acid), a solution of a mixture of basic ferric chloride and 
sulphate (from the oxidation of ferrous sulphate by nitric acid 
and hydrochloric acid), a basic ferric chloride solution, and a 
basic ferric acetate solution. He employed a strength of iron 
liquor containing i per cent. FcaOg of the weight of the pelt in 
a little over four times the weight of water of the weight of the 
pelt. According to him 7.88 per cent, of FegOg in the leather 
on the basis of the dry weight is sufficient to convert the pelt into 
leather. He stated that he had started with a tan liquor having 
such a basicity as to correspond to the formula.Fe2(S04)2(OH)2, 
but his iron and basicity determinations in the liquor showed that 
the liquor he used was more acid than this, the basicity of his 
first liquor (basic ferric sulphate) being only one-half of this 
value, and that of his second liquor (a mixture of basic ferric 
sulphate and chloride) less than a half of this value. We have 
^* Le Cuir, Aug. i, Sept. I and Sept. 15, 1919. 



IRON tannage; 13 

found that a sulphate Hquor having so high a basicity as to cor- 
respond to Fe2(S04)2(OH)2 is too alkahne for use. Throughout 
the course of tanning he strengthened the hquor with fresh por- 
tions of the strong Hquor, He drew a conclusion that the final 
basicity of the liquor was the same as that at the beginning of 
tanning — a conclusion that has not been confirmed by our experi- 
ments. 



14 LEATHER CHEMISTS ASSOCIATION 

Section III. Investigation of Different Methods of Oxida- 
tion AND Some Methods of Preparation of 
Iron Liquor From Copperas. 

As the largest and cheapest commercial source of iron salts is 
in the form of the ferrous sulphate or "copperas," FeS04.7H20, 
this particular salt of iron naturally forms the starting point for 
the preparation of the tanning solutions. As the ferric salts are 
generally more expensive than the corresponding ferrous salts, 
economy demands that the iron tanning liquor shall be made from 
the ferrous salt — "copperas" in particular — rather than directly 
from a ferric salt purchased as such. Hence it is clear that a 
proper method of oxidation is essential to the preparation of this 
tanning liquor and constitutes one of the main factors in the 
economic aspects of iron tannage. Consequently it is worth while 
to devote some attention to the study of different methods of 
oxidation and of the value of different oxidizing agents from the 
tanning point of view. In this investigation, mostly qualitative, 
we have constantly kept in view three points, namely (i) the 
simplicity of the method by which oxidation can be carried out; 
(2) the character (acidity, etc.) of the liquor thus obtained in 
regard to the convenience. for use; and (3) the cheapness of the 
chemicals employed. In the following there is given a brief 
summary of the properties and behavior towards ferrous sulphate 
solution of some of the more important oxidizing agents, although 
the study includes practically all of the ordinary oxidizing agents 
available. 

Sodium Dichromate, Na2Cr207.2H20. — Oxidation goes on in 
the cold and to completion (as tested with K3Fe(CN)6 solution) 
even in the absence of any acid added. There is a tendency for 
the ferric oxide, FcaOg.xHgO, to separate out. With a small 
amount of a mineral acid added no precipitate will be formed 
and the reaction is distinctly accelerated by the higher hydrogen- 
ion concentration. 

CrsO^^ + 6Fe++ + 14H+ «^> 2Cr+++ -f- 6Fe+++ -f 7H2O. 

This method is an important one and embodies one mode of 
tannage found to give satisfactory results. It has several advan- 
tages: (i) that the oxidation reaction requires only a very low 



IRON tannage; 15 

hydrogen-ion concentration, so that the acidity of the Hquor ob- 
tained is entirely in control; (2) that the oxidation potential is 
high and the oxidation reaction is completed very smoothly in the 
cold; and (3) that the waste product, Cr+++ salts left in the 
resulting liquor is itself a valuable tanning agent and constitutes 
what may be called a co-tanning agent with the iron. Further- 
more, a slight excess of the sodium dichromate in the liquor 
could effectively prevent any ferric iron from being reduced to 
the ferrous state in the course of tanning. In spite of the present 
high price of the dichromate, the process has merits of its own 
as will be presented in detail in a later section. 

Sodium Chlorate, NaClOa. — The oxidation by a chlorate 
NaClOj or KCIO3 in a cold solution does not occur without the 
addition of a mineral acid (HCl or H2SO4). On adding the acid 
the reaction takes place rapidly and goes to completion in the cold. 
The solution assumes a greenish-yellow color, probably due to 
some chlorine dioxide formed, CIO,. 

2NaC103 -f HCl *»^ 2NaCl + HCIO3. 

HCIO3 + 6FeSO, + 5HCI ^^ 2Fe2(SOj3 + 2FeCl3 + 3H2O. 

3HCIO3 ^»*^ H2O + 2CIO2 + HCIO4. 

With a weaker acid, like acetic acid, oxidation takes place on 
heating, giving a red solution due to the formation of the basic 
ferric salt. Without any acid added the reaction can be brought 
about by heating, but Fe203.xH20 would then be thrown down. 
As the chlorate is rather expensive, especially the potassium chlor- 
ate, the process will not be economical, although W. Mensing 
advocated its use in his patent.^^ 

Manganese Dioxide, MnOg. — In the absence of any acid, no 
reaction takes place, MnOj being insoluble. On adding HCl, 
evolution of CU gas is observed and the solution turns yellowish, 
this being the characteristic color of the ferric chloride in 
solution, 

MnOa -f 4HCI «-► MnCl^ + Cl^. 
The reaction proceeds to completion in the cold. 

If in place of HCl, H2SO4 is used, the yellow color does not 
develop. The reaction goes to completion only when a large 

" Swiss Patent No. 75,775, Class 40, February i, 1918. 



i6 i,e;ather chemists association 

excess of MnOg and H2SO4 is employed. A small but distinct 
amount of the permanganate is formed when all ferrous iron has 
been oxidized. Hence if MnOg is to be employed as the oxidiz- 
ing agent, HCl rather than H2SO4 should be used. Molac and 
Friedel in 1855 prepared their iron liquor from ferrous sulphate 
with MnOa and HgSO^. 

Nitric Acid, HNO3.— i. HNO^ alone." Dilute HNO3 has 
scarcely any oxidizing action upon a dilute FeS04 solution in 
the cold. With somewhat more concentrated HNO3 solution, a 
black coloration gradually develops due to the reduction of some 
HNO3 to nitric oxide, NO, which unites with FeSO^ to form 
the ferrous nitroso compound FeSO^.NO. The black color deep- 
ens on warming and persists even on boiling if the concentration 
of the HNO3 in the solution is not high enough to effect the oxi- 
dation. With the addition of more HNO3 or with an increase 
in the concentration of the HNO3 due to the loss of water by 
prolonged boiling, complete oxidation finally takes place and all 
of the nitroso compound is decomposed. The solution then boils 
violently, brown fumes of nitrogen dioxide, NO2, being given 
off. During the evolution of the gas the temperature of the solu- 
tion falls 4° or 5° C. The amount of HNO3 required for com- 
plete oxidation depends largely upon the concentration rather 
than the absolute quantity of HNO3 present. Starting with a 
saturated solution of FeS04.7H20 (one part of FeS04.7H20 in 
about one and a half parts of water by weight) and using 1.42 
HNO3, a considerably less amount of the HNO3 need be em- 
ployed, but 25 per cent, of 1.42 HNO3 of the weight of FeSO^.- 
7H2O taken is found to be the working minimum under such 
conditions. Although complete oxidation can still be brought 
about with a less quantity, say 20 per cent., the liquor obtained 
becomes too alkaline and has a muddy appearance, due to the 
separation of the ferric oxide. This can be readily understood 
when we see that when the ferrous salt is oxidized to the ferric 
salt, the solution becomes less acid and the greater part of the 
HNO3 used simply goes to furnish the necessary acidity (see 
paragraph under HNO3 -j- HgSO^ below). Some HNO3 is lost 
by boiling. 

"This method was used by Knapp, but the Hquor'he prepared yielded 
much Fe203.xH20. 



IRON TANNAGE IJ 

The oxidation of the ferrous iron to the ferric state by HNO3 
in the cold can only approach completion when a very large excess 
of the concentrated HNO3 is added to a concentrated ferrous sul- 
phate solution. Consequently boiling is a necessary operation 
which makes the process less simple, as boiling nitric acid solu- 
tion requires a special container to resist corrosion. 

2. HNO^ -\- H2SO4. As was said above, the oxidation of the 
ferrous solution to the ferric state renders the solution more 
alkaline, so that an acid must be added to prevent any ferric salt 
from being hydrolyzed and precipitated. If H2SO4 in the re- 
quired amount is added to supply the acidity, the HNO3 needed 
can be cut down from 25 per cent, of the weight of the FeSO^.- 
7H2O to 9.5 per cent.. Thus — 

3FeS04.7H20 + 4HN03«->3Fe(N03) (SOJ + NO + 2H2O 
theoretically requires 30.2 per cent. HNO3 of FeS04.7H20. 
6FeSO,.7H20 -f 2HNO3 + 3H2SO4 ^h- 

3Fe2(SOj3 + 2NO+4H20 
theoretically requires only 7.55 per cent. HNO3 of FeS04.7H20. 
As HNO3 is far more expensive than H2SO4 this method is more 
economical. 

3. H^SO^ -\- NaNO^}'^ — With a saturated ferrous sulphate solu- 
tion (i part FeS04.7H20 in about 1^2 parts of water) and with 
25 per cent, of HgSO^ and NaN03 each in excess calculated ac- 
cording to the following reaction, 

6FeS04.7H20 -f 4H2SO4 + 2NaN03 «^ 

3iFe2(SOj3 + Na2S04 -f 2NO + 4H2O, 
the reaction goes on very smoothly by continued boiling. The 
resulting liquor, thick like a syrup and dark red in color, has a 
specific gravity as high as 1.50 — 1.55 and contains iron as 
Fe^C 504)3 from 40-45 per cent. As Chile saltpetre, NaNOs, is 
much cheaper than HNO3, this method is still more economical. 
Furthermore, the Na2S04 formed in the liquor cuts down the 
amount of NaCl needed for tanning. Taking 4-5 per cent, as 
the normal figure for NaCl used on the weight of the pelt, this 
saves about 25 per cent, of NaCl required. 

4. HNO. -|- HCl, aqua regia. — The reaction starts in the cold. 
When only a small quantity is added to the ferrous sulphate solu- 

" This method was used by Knapp, but the details differ. 



i8 i,eathe;r che;mists association 

tion the black color of the nitroso compound is observed, but on 
further addition of aqua regia the nitroso compound is decom- 
posed and the reaction goes to completion, although the end- 
point is not very permanent. The solution on standing gradually 
assumes a golden yellow color due to some hypochlorous anhy- 
dride CI2O formed. The main reactions seem to be — 
HNO3 -f 3HCI «^ NOCl -f CI2 + 2H2O, 
NOCl + C\,-+ 3FeSO, «^ NO -f FeClg -f Fe^CSOJa, 
although other oxidizing agents such as HNOg and HCIO are 
also formed. The proportion of 1.20 HCl to 1.42 HNO3 is 3- — 3.5 
to I by volume of the concentrated solutions. 

The oxidizing power of aqua regia seems to be greater than 
that of the concentrated HNO3 alone, but some assert that there 
is no difference in oxidation potential between aqua regia and 
concentrated nitric acid.^^ As aqua regia is difficult to handle and 
rapidly corrodes the container, this method of oxidation is neither 
economical nor simple. 

It will be noticed that with the possible exception of HNO3 -|- 
HCl, all methods involving oxidation by HNO3 in some form 
require a boiling temperature. This constitutes a very unfortu- 
nate feature. There are, however, soine distinct advantages in 
the case of NaN03 -|- H2SO4, vis., ( i ) that the materials used are 
cheap; (2) that a very concentrated liquor can be obtained, and 
(3) that with a proper amount of H2SO4 employed the liquor 
obtained is stable and there is no danger of deterioration on 
storing. 

Chlorine Gas, CI2. — The oxidation by chlorine is very smooth 
and simple. The reaction starts in the cold and goes to completion 
when the gas is passed in under a small pressure and when effi- 
cient stirring is maintained. The reaction furnishes its own 
acidity and in the right proportion, 

CI2 + H2O »^ HCl -f HCIO. 
HCIO + FeSO^ + HCl ^> Fed. (SO J -f- H2O. 

The process is very efficient and incurs practically no loss of CU 
if two or three units are connected in series and the solutions 
treated countercurrently. Iron liquor obtained by this method is 
"Moore, "Aqua Regia," /. A. C. S., p. 1091 (1911), and "Aqua Regia 
II," /. A. C. S., p. 33 (1913). 



IRON TANNAGE 19 

of course saturated with Clg and so contains a slight excess of it, 
but this is indeed an advantage for it prevents the ferric sah 
from being reduced again and also enables the liquor to be kept 
in storage without any danger of deterioration, i. e., either 'Fe^O^.- 
xHgO separating out or some ferric salt changing to the ferrous. 
As liquid chlorine now can be obtained in large quantities and at 
a reasonable cost, there is in it much to recommend from a com- 
mercial standpoint. 

Bleaching Pozvder, CaCl.ClO. — This method is one of the first 
used by us in this research. The oxidation goes on in the cold. 
With large excess it is possible to oxidize completely the ferrous 
iron without adding any acid, in which case a precipitate of 
FegOa.xHsO is liable to come down. With a weak acid present, 
e. g., acetic acid, the reaction is accelerated, and with a mineral 
acid it goes to completion readily. CaS04.2HoO is thrown down. 
FegOa-xHaO is more readily separated from this liquor probably 
due to the greater coagulating influence of the divalent radicals. 
The bleach suspension itself reacts alkaline so that the addition 
of an acid is rendered more necessary. As the oxidizing agent is 
really HCIO, there is no advantage in using this material over 
liquid chlorine and the cost is greater for the chlorine content in 
this form. The bleach, however, can be used to advantage in 
connection with iron tanning processes (to be described in later 
Sections). 

Sodium Nitrite, NaNO,. — Unlike NaNOg, oxidation begins in 
the cold, but basic ferric salt would be precipitated when no acid 
is added. On adding an acid (HCl or H2SO4) the red precipi- 
tate redissolves and nitrogen oxide gases are rapidly given off. 

2HNO2 ^m^ H2O + N2O3. 
Nads ^ NO2 + NO. 
The reaction then goes to completion in the cold when an excess 
of NaNOg is present. With acetic acid the reaction can also go 
to completion giving a colloidal suspension. As the NaNOa solu- 
tion reacts alkaline, the addition of an acid is all the more neces- 
sary. The oxidation potential of HNO2 is lower than that of 
HNOg,^^ although the latter produces no appreciable oxidation 
in the cold, especially when the solution is dilute. Since HNO, 
"Ihle, Z. f. phys. Chem., Vol. 19, p. 577 (1896). 



20 i,e;ather chemists association 

is unstable and is decomposed readily, the loss of HNOg through 
decomposition and volatilization is very great. Bystron and Viet- 
inghoif^° patented a cyclic process to collect these gases, oxidize 
them all to NOg by air, and use the gas again for oxidation. It 
is probable that such a method can not be carried out in practice 
without undue complications. 

Hydrogen Peroxide, HgOg. — The oxidation starts in the cold, 
but does not go to completion without a large excess. Even with 
a large excess the end-point is not stable. FegOg.xHgO is thrown 
down on standing for a few minutes. The solution is red, due 
to the basic ferric salt formed. If an acid (HCl or H2SO4) is 
added the color of the solution becomes yellow and the reaction 
goes to definite completion, giving a more permanent end-point. 

H,02 -f 2FeSO, -f H2SO, ^m^ Fe^ ( SO,) 3 + 2H,0. 
As H2O2 cannot be obtained cheaply, at least at present, its use 
will not be commercially practicable. 

Potassium Per chlorate, KCIO4. — The oxidation starts only on 
warming when no acid is added, but the solution then becomes 
turbid. On adding an acid, the turbidity clears up but the reac- 
tion does not go to completion even when the solution is heated 
to boiling. Thus contrary to what one might suppose, the oxida- 
tion potential of the perchloric acid is lower than that of the 
chloric acid. 

Oxygen From the Air. — The oxidation of the ferrous sulphate 
by air oxygen would be a very cheap method if it could be brought 
about rapidly enough. Fine bubbles of air are passed through the 
ferrous sulphate solution, but the reaction is too slow, only 6 per 
cent, being oxidized at the end of four hours. At an elevated 
temperature, 8o°-90° C, the reaction is more rapid, but even then 
only 12 per cent, is found to be oxidized in four hours. 10 per 
cent, concentrated H2SO4 of the weight of the FeS04.7H20 in 
the solution (i part FeS04.7H20 to 2 parts water) should be 
present, otherwise ferric oxide would be precipitated. The ordi- 
nary catalytic agents, such as the mercuric salt (5 per cent. 
HgClg) and the phosphate (5 per cent. Na2HS04) do not seem 
to help. Ozonized air, or air led through an electric ozonizer, 
would do better, but would be expensive. 

*• German Patent No. 255,320 (1911) and a number of patents fol- 
lowing. 



IRON TANNAGE 21 

Anodic Oxidation. — Oxidation by electrolysis seems to have 
some possibilities. The anode is best made of lead but the cathode 
can be lead, graphite, copper, or even iron. These materials have 
been tried. Lead and graphite as cathodes are inert in the acid 
(H2SO4) solution and are found to be suitable. Copper cathode 
is not attacked by tlie acid when the cells are running, but when 
the current is stopped it is attacked by the acid with the aid of 
atmospheric oxygen, thus contaminating the electrolyte. An iron 
cathode can be used to advantage as it is of the same material as 
the liquor is composed of. As the cathode environment is a 
reducing one, a diaphragm such as porous earthware, asbestos 
felt, or electro-filtrose should be provided to separate the cathode 
portion from the main electrolyte. This arrangement prevents 
the hydrogen gas evolved at the cathode from mixing with the 
main electrolyte. The cathode chamber need not be large, and a 
capacity of about one-fifth or less of the volume of the main 
electrolyte is sufficient. The cathode solution can best be a plain 
H2SO4 solution and should be maintained at a higher level than 
the body of the electrolyte to prevent diffusion of much iron into 
the chamber. It is found that the H2SO4 concentration inside 
this chamber should be maintained high, about two or three times 
as high as in the main electrolyte, otherwise some iron might be 
plated on the cathode. The electrolyte is made by dissolving cop- 
peras in about twice its weight of water and adding 15 per cent, 
concentrated HgSO^ of the weight of copperas taken. As, gen- 
erally speaking, the oxygen over-voltage is low-^ oxygen gas is 
easily caused to be discharged at the anode, resulting in low cur- 
rent efficiency. To prevent this, the anode current density must 
be low, i. e., the anode surface must be large. It is found that with 
the anode current density of 0.20 — 0.40 amperes per square deci- 
meter for a liquor containing 25 — 40 per cent, of copperas, the 
over-all current efficiency is as high as 70 — 75 per cent., even when 
there is no circulation or stirring in the main electrolyte. With 
good circulation or stirring and with a concentrated electrolyte (30 
— 40 per cent. FeS04.7H20) a higher current density can be 
safely employed without any danger of the discharge of oxygen 
gas at the anodq. The cell takes a voltage of 2.4—3.0 volts depend- 
ing upon the distance between the electrodes, the condition of the 
^' See Allmand, "Applied Electrochemistry," p. 144 (1920). 



22 I^EATHER CHEMISTS ASSOCIATION 

diaphragm, the concentration of the electrolyte and its tempera- 
ture, but with the cells running properly and with the electrodes 
about 4 inches apart, this terminal potential drop should not be 
much over 2.6 volts under normal conditions. The back E. M. F. 
is approximately 2 volts. Oxidation can go to completion by this 
method, but the end-point is not quite permanent. As each cell on 
the average takes less than 3.0 volts, there can be connected in 
series on the 120 main about 40 cells. In this way, the method 
compares favorably with the cheapest chemical methods. The 
disadvantage seems to be that considerable amount of FcgOs-xHaO 
is thrown down as sludge in the cells and the cells need close 
attention and regulation in regard to the acidity in the cathode 
chamber, proper conditions of the diaphragm, etc., otherwise sec- 
ondary reactions might set in and the cells fail to function prop- 
erly. It is found that the lead anode is oxidized only after all 
iron has been oxidized. 

From the above brief description it will be seen that, to pro- 
duce such a cheap product as the ferric salt, many of the costly 
and rarer oxidizing agents will find no place. Considerations of 
the different factors point, for the present at least, to the methods 
oxidation by chlorine, oxidation by NaNOg and H2SO4, oxida- 
tion by HNO3 and HgSO^, and oxidation by ^^^Q.x^O^ utilizing 
the chrome. The other methods that possess some possibilities 
are anodic oxidation and oxidation by the atmospheric oxygen in 
some form, while oxidation by NaClOg or other chlorate, oxida- 
tion by NaNOa, and oxidation by MnOa and HCl seem to have a 
doubtful economic value. 

The details of a few methods of preparation of the iron liquor, 
which have been found suitable, will now -be given. They are 
based on the oxidation by (I) liquid chlorine, (II) sodium nitrate 
and sulphuric acid, (III) nitric acid and sulphuric acid, and (IV) 
sodium dichromate. Liquid chlorine, as far as we know, has 
never been used before. While sodium nitrate and nitric acid 
used for oxidation in conjunction with sulphuric acid are more or 
less well known"- the details of procedure in regard to the propor- 
tions of the materials employed, the concentration aimed at and 
acidity desired, etc., are worked out independently. The condi- 

'' H2SO4 and HNO3 used as early as 1842 by Bordier, and H2SO4 and 
-NaNOs in 1879 by Knapp. 



IRON TANNAGE 2^ 

tions as given here are those found capable of producing (i) a 
high concentration of iron in the liquor, (2) complete oxidation 
of iron with some excess of the oxidizing agent in the liquor, (3) 
complete reaction involving the use of a minimum amount of the 
oxidizing agent and other materials, and (4) a degree of acidity 
as near that suitable for tanning operation as possible, consistent 
■with the stability of the liquor. The one difficulty with the pre- 
pared sulphate liquor is that, unless the degree of acidity is above 
a certain minimum it does not keep zvell and ferric oxide is liable 
to separate out. The separation of much ferric oxide would 
greatly impair the tan liquor and this danger should always be 
guarded against when the liquor is to be placed on the market 
where its keeping quality is of vital importance. 

(I) Oxidation by Chlorine. — For laboratory preparation. A 
desired weight of commercial copperas is placed in 1%. to 1^/2 
times its weight of water in a large container provided with an 
entrance and an exit hole. Through one hole is passed a delivery 
tube reaching to the bottom of the container. During the passage 
of the gas, the contents are stirred constantly. As the copperas 
crystals gradually disappear more can be added until the total 
weight of the copperas used is equal to the weight of the water 
present. Toward the end, the exit hole is stopped temporarily to 
create a small pressure of the chlorine gas above the solution. 
The completion of oxidation is tested with K3Fe(CN)(. solution. 
The end-point should be so permanent that a test sample with 
K3Fe(CN)6 solution should be colored deep red and remain so 
for at least one-half hour in contact with air. 

To the liquor 35 per cent, commercial NaCl and 10 per cent, 
soda ash of the weight of the copperas taken are added, the latter 
being first dissolved in a small quantity of water and added very 
slowly with stirring. This liquor thus neutralized should be used 
without much delay. To make the liquor keep for a short period, 
pa^s in again chlorine gas under a small pressure and immediately 
stopper the bottle tightly. For long storage, it is safer not to add 
this quantity of NagCOa until ready for use. The bottle should 
be tightly closed so that no chlorine gas can escape. No acid 
need be added to the ferrous sulphate solution for chlorine oxida- 
tion. 



24 IvlBATHEjR CHEMISTS ASSOCIATION 

For commercial preparation, cast iron or enameled iron tanks 
may be used. Two or three units sould be connected in series 
and the gas passed in countercurrently. 

The liquor thus prepared is dark red in color, but should be 
absolutely clear, and should remain so without depositing yellow 
hydrated ferric oxide on standing. It is rather thick and, after 
the addition of NaCl, has a specific gravity of 1.39. It contains 
approximately 32 per cent, of iron calculated as Fe2( 804)3. 

On the basis of 100 pounds of the drained pelt, the cost of 
preparation is estimated as follows : 

Copperas 14 pounds at i^ a pound $0.14 

Liquid chlorine 2 pounds at 7^ a pound .14 

NaCl (crude) 5 pounds at ^^ a pound .02j^ 

Na2C03 (comm.) ij4 pounds at lYz^ a pound .02^ 

Total $0.32^4 

This will give approximately 2^ gallons of the liquor in a 
concentrated form, weighing about 31 pounds. For use, dilute 
to 15 — 25 gallons for drum tannage. The cost of preparation per 
gallon of the concentrated liquor is estimated to be about 12 cents, 
or per pound a little over i cent. 

(II) Oxidation by NaNO^ and H^SO^. — For laboratory prep- 
aration. Take a desired quantity of commercial copperas and 
place it in a large container containing about ij^ times its weight 
of water to which have been added 30 per cent, of 66° Be. H2SO4 
and 12^ per cent, of Chile saltpetre of the weight of the cop- 
peras taken. Heat to boiling and boil gently until brown fumes 
of NO2 are finally given off. Remove the burner during evolu- 
tion of the gas. 

Add 20 per cent. NaCl and neutralize slowly with gj^ per cent, 
soda ash previously dissolved in a small quantity of water. The 
liquor is ready for immediate use. 

For Commercial Preparation. Use an enameled open kettle* 
provided with a steam jacket taking exhaust steam. The kettle 
should have a somewhat larger capacity, as during evolution of 
the gas the liquor foams badly. 

The evolution of NO2 fumes indicates the end-point for the 
reaction and that a small excess of HNO3 is present. This rep- 



IRON tannage; 25 

resents, of course, a loss in HNO3 though small, which would 
otherwise be available for oxidation. But as a small excess is 
always necessary to carry the reaction to completion, this minor 
loss seems to be unavoidable. 

The above proportion of NaNOg and H2SO4 represents an 
excess of 15 — 20 per cent, over the theoretical quantity in each. 
This is found to be the minimum quantity, especially in the case 
of NaNOa, in order to secure a complete oxidation without ren- 
dering the resulting liquor too alkaline. Rather prolonged boil- 
ing is needed before NO2 fumes are given off, as a certain con- 
centration of HNO3 in solution must be attained before the com- 
plete reaction can take place. But the liquor obtained is more 
concentrated due to the loss of water by boiling. 

Before the addition of NaCl and NagCOg the liquor has a 
specific gravity of about 1.50 containing, in this condition, about 
36 per cent. Fe2( 804)3. It is a thick liquid, dark red in color. 
It is absolutely clear and should remain so on long standing with- 
out deposition of ferric oxide. 

On the basis of 100 pounds, the cost of manufacture is esti- 
mated as follows : 

Copperas 14 pounds at i^ a pound $0.14 

H2SO4 (66° Be.) 4V5 pounds at i^ a pound .04V3 

Chile Saltpetre 1% pounds at 3^ a pound .0554 

NaCl 3 pounds at J/2^ a pound .01 ^ 

Soda ash ij/^ pounds at i^^ a pound .02^ 

Total $0.2775 

This gives approximately 2.5 gallons of the concentrated liquor 
weighing about 30 pounds before neutralization with soda ash. 
For use, dilute the liquor to 15 — 25 gallons for drum tannage. 
The cost of preparation is estimated to be about 11 cents per gal- 
lon of concentrated liquor, or about i cent per pound. 

(Ill) Oxidation by HNO^ and H^SO^. — For Laboratory Prep- 
aration. Take a desired quantity of copperas and place it in a 
container containing i^^ times its weight of water to which have 
been added 9^-10 per cent, of 1.42 HNO3 and 19 per cent, of 
66° Be. H2SO4. Proceed as in (II). 35 per cent. NaCl of the 
weight of the copperas taken and 9}^ per cent, soda ash are later 
added. 



26 i^EATHER che;mists association 

The use of HNO3 alone without sulphuric acid is very expen- 
sive and wasteful. 

The liquor has the same general properties as that prepared by 
(II). 

On the basis of 100 pounds of the hide, the cost of manufacture 
is as follows : 

Copperas 14 pounds at 1^ a pound $0.14 

HNO3 (1.42) I J^ pounds at 8^ a pound .20^ 

H2SO4 (66° Be.) 2% pounds at i^ a pound .02^ 

NaCl 5 pounds at 3^^ a pound .02 J/^ 

Na2C03 I J^ pounds at I J/^^ a pound .02 

Total $0.3 1 Vs 
The resulting liquor is of about the same volume as that in 
(II) but weighs a little less. The cost of preparation is estimated 
to be about 13 cents per gallon of concentrated liquor, or a little 
over I cent per pound. 

(IV) Chrome-Iron Liquor. — For Laboratory Preparation. Take 
a desired quantity of copperas and place it in an equal weight of 
water. Add 35 per cent, of 66° Be. H2SO4. Stir until as much 
of the copperas is dissolved as possible. Cool and add gradually, 
with stirring and cooling, 20 per cent, of sodium dichromate 
crystals of the weight of the copperas taken, and then add 30 per 
cent. NaCl. The liquor is ready for use without neutralization, 
or with but a small quantity of an alkali added. 

For Commercial Preparation. The proportions and procedure 
hold good, except that an enameled tank or crockery should be 
used and cooling coils of hard lead should be provided". 

Considerable heat is evolved upon the addition of H2SO4 and 
in the introduction of Na2Cr207.2H20, so that cooling facilities 
should be provided in commercial work when a concentrated 
liquor is desired. When the liquor is made by the tanners them- 
selves for immediate use, the following procedure can be adopted : 
For each 100 pounds of the drained skins, take 10 pounds of cop- 
peras in 40 pounds or 5 gallons of water, contained in crockery 
ware. Add ^Vs pounds of 66° Be. H2SO.1, stir well and cool, 
and then gradually introduce 2 pounds of Na2Cr207.2H20 with 
good stirring. Dilute to 15 — 25 gallons for drum tannage. No 
alkali need be added. 



IRON TANNAGE 2/ 

The liquor is rather thick and appears black, as the red color 
of the iron and the green of the chromic salt tend to neutralize 
each other. After the addition of NaCl, it has a specific gravity 
of 1.49 and contains about 26 per cent, iron calculated as Fcj- 
(804)3 and Sy2 per cent, chromium as CrgC 804)3. 

On the basis of 100 pounds of drained pickled sheepskins, the 
cost of preparation is estimated as follows : 

Copperas 10 pounds at i^ a pound $0.10 

HaSO^ (66° Be.) ... a'A pounds at i^ a pound .03V5 

NajCr20T.2H20 2 pounds at 28^ a pound .56 

NaCl 3 pounds at J^^ a pound .oiJ/4 

Total $o.7oVio 
The liquor obtained in this concentrated form measures approx- 
imately 2.2 gallons and weighs 2yy2 pounds. The cost of prepar- 
ation is about 32 cents a gallon, or 2.6 cents a pound. 

This liquor is different from any of the above in that it con- 
tains a basic chromic salt as well as ferric salt, which chromic 
salt also contributes to the tannage. While it costs two and a 
half times as much as the liquor obtained by any of the first three 
methods, its cost is still very much less than that of a one-bath 
chrome liquor. 



28 lsathe;r chemists association 

Section IV. HydroIvYsis and Decomposition of Ferric and 
Chromic Saints Compared. 

It may wdl be suspected that one of the reasons why iron liquor 
is so much more difficult to manage than chrome liquor might be 
found in the greater tendency of the ferric salts, particularly the 
sulphate, in solution to yield readily a precipitate. On the other 
hand, it may be appreciated why chrome liquor is advantageous 
when the peculiar properties . of the chromic salts are recalled. 
Chromic salts, both the sulphate and the chloride, in solution are 
capable of forming "complexes." The bluish hexahydrate chro- 
mic chloride, CrCl3.6H20, obtained from crystallization in the 
cold, gives a violet solution which turns green on boiling. This 
saturated violet solution, when treated with hydrogen chloride gas 
in the heat yields green crystals, the aqueous solution of which, 
when freshly prepared, possesses only two chlorine atoms out of 
the three in the molecule that are precipitable by AgNO^.^^ Chro- 
mic sulphate in solution also forms "complexes." The reddish- 
violet crystals, Cr2( 804)3.151120, gives a bluish violet solution in 
cold water which also turns green on boiling. This green com- 
pound can yield, with H2SO4 on warming, products whose S04= 
radical is not precipitable by BaCl2."* The chemistry of chromic 
salts in solution is certainly a complex one. Certain irregularities 
in the behavior of a chrome liquor may possibly be due to this 
complex nature. As far as is known, no such peculiarities exist 
in the ferric salt solution. 

A qualitative comparison between the behavior of a neutral 
ferric salt solution and a neutral chromic salt solution both in an 
excessively diluted form is instructive, i cc. of a 10 per cent, 
neutral ferric sulphate solution is diluted to 100 cc. and allowed 
to stand for 24 — 36 hours. A fine precipitate of yellow hydrated 
ferric oxide copiously settles at the bottom. With further dilu- 
tion to 300 cc. and standing, about two-thirds of the original 
amount of iron in solution separates out and the solution becomes 
so depleted that it appears almost colorless. When diluted to 
1,000 cc. and allowed to stand for three to four weeks, the solu- 
tion gives only a slight blue coloration with K4Fe(CN)6. If a 

^ See Ostwald, "Principles of Inorganic Chemistry," translated by A. 
Findlay, p. 636 (1914). 

^*Ibid. Also HoUeman, "A Text Book of Inorganic Chemistry," trans- 
lated by H. C. Cooper, p. 461 (1916). 



IRON tannage: 29 

normal chromic sulphate is dissolved and diluted to the same de- 
gree, only a slight turbidity is observed, but very little, if any, 
precipitate separates out after 24 hours' standing and the solution 
remains light green. This comparison shows in a qualitative way 
that the chromic salt in solution is very much less susceptible of 
hydrolysis and decomposition with dilution. 

Next, a mixture of a basic ferric sulphate and basic chromic 
sulphate solution was quantitatively studied. This solution is 
prepared by placing 50 g. of ferrous sulphate crystals in 50 cc. 
of distilled water containing 17.4 g. C. P., 1.84 HoSO^ and then 
gradually introducing 9 g. of sodium dichromate crystals, Nag- 
Cr207.2H20 to the mixture. Upon introduction of the sodium 
dichromate, ferrous sulphate crystals are dissolved, much heat is 
given off, and a thick dark liquid containing a mixture of the basic 
ferric sulphate and chromic sulphate in solution is obtained. This 
solution appears black by reflected light and dark red by trans- 
mitted light. It has a specific gravity of 1.46. Analysis shows 
that it contains, per 10 cc. — 

0.07312 equivalents of SOi^ (from acidity determination). 

0.06820 equivalents of Fe+ ++. 

0.021 17 equivalents of Cr+ ++. 
The ratio of the number of equivalents of the sulphate radical 
SO^^ (which is divalent) to that of Fe and Cr combined (each of 
which is trivalent) is 0.819 : i.ooo, so that there are not enough 
sulphate radicals to go with iron and chromium in the solution. 
This condition may be summarized in the formula : 

Radical M'" : 80,= : (0H-) (M"' = Fe+ + + + Cr+ ++) 

Ratio of equiv. i.ooo: 0.819 : 0.181 (by difference). 
The idea of using such a quantity of H2SO4 and making a liquor 
as concentrated as this needs some explanation. In the first place, 
if a liquor is to be placed on the market it has to be in as concen- 
trated a form as practicable in order to save freight charge in 
transportation and to avoid inconvenience in handling. In the 
second place, a tanning liquor must always contain some degree 
of basicity. Too much acid left in the liquor not only means just 
so much alkali needed for neutralization before the tanning oper- 
ation, but also involves a danger of not getting the proper degree 
of basicity for tanning in the hands of men who are not quite 



30 i,e;ather che;mists association 

familiar with chemistry. We find by a number of experiments 
that this proportion of the sulphuric acid represents the minimum 
quantity that should be present in order to make the liquor 
alkaline enough to be used for tanning without neutralization 
(Chrome-Iron Joint Tannage) and yet acid enough to make the 
liquor keep for a considerable period without danger of precipi- 
tation. 

A quantity of this concentrated liquor measured through a bur- 
ette is diluted to different volumes with distilled water, and the 
solution allowed to stand for 24 — 48 hours. The supernatant 
liquid is filtered and pipetted for analysis. Chromium is deter- 
mined by NagOa oxidation and by titration against sodium thio- 
sulphate solution using KI and starch as indicator. Ferric hy- 
droxide that is precipitated by NagOa in the same sample is fil- 
tered off,* washed and dissolved from the filter with hot dilute 
HCl solution. The amount of iron in this solution is determined 
by the Zimmermann-Reinhardt method. These results are tab- 
ulated as follows : 

* When much iron is present, a considerable amount of the chromate 
is absorbed by the ferric hydroxide precipitated from the peroxide oxida- 
tion, so that the result of the chromium determination in the filtrate is 
always too low while the iron by the Zimmermann-Reinhardt method 
becomes high. To avoid this error, the supernatant chromate solution after 
the peroxide oxidation is decanted through a filter and the ferric hydroxide 
left in the beaker is dissolved by adding a small amount of hot dilute HCl. 
The solution is now diluted and the ferric hydroxide re-precipitated with 
an alkali at a boiling temperature. A small amount of Na202 may be 
introduced with the alkali, in which case care must be taken to decompose 
all the peroxide again. If a large quantity of chromium is also present, 
a second re-precipitation is necessary in order to remove all the chromium 
from the ferric hydroxide. 



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IRON TANNAGE 



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It is evident that the extent of hydrolysis increases with dilution. 
Fe,(SOj3-2Fe+++ + 3SO,= 

+ + 

6H2O ^ 60H + 6H+ 

11 11 

2Fe(OH)3 3H.SO, 

11 
FegOg.xHgO (yellow crystalline ppt. of 
dehydrated ferric hy- 
droxide). 
This is strictly according to the Mass Action principle, for a 
greater dilution means a greater active mass of water and hence 
the following reaction is pushed to the right : 

Fe,(SOj3 + 6H,0 - 2Fe(OH)3 + sH.SO,. 
Another way of interpreting this is that with greater dilution the 
hydrogen ion concentration is correspondingly lowered. That is 
to say, the alkalinity of the solution is increased and consequently 
the degree of hydrolysis is increased. 

The above results show how ferric sulphate in solution is more 
readily hydrolyzed and decomposed than, the chromic sulphate. To 
show this more clearly the following curves are plotted: 

F/G.-J 

Curves Showing Fe2 (50^3 and Crz (50^3 
/Remaining Jn Solution ys. Dilution 




I z 

Lo^. of Number of Times of Dilution 



IRON TANNAGE 



33 



F/c-JI 

ffaf/'o of FezO^ to CrzO^in Solution 






"I^TT^ ^ 



12 3 

Log. of Number of Times of Dilution 

The precision of the above determinations is not better than ^ — i 
per cent. For, when the precipitate, especially in the last two solu- 
tions, separates out abundantly in a fluffy manner and only the 
supernatant clear liquid is taken for analysis, the original volume 
ratio does not exactly hold, but the error is small and can be 
neglected. 

A mixture of the corresponding chlorides, namely the ferric 
chloride and the chromic chloride, was taken and similarly studied. 
The basicity relation as determined by analysis was as follows : 

Radical M"' : CI" : OH" (M"' = Fe+ + + + Cr+++) 

Ratio of equiv. I. loo: 0.800 : 0.200 (by difference). 

Dilution in much the same way as in the corresponding sulphate 
mixture was carried out. In no case was there any precipitate 
observed, not even where the original solution of the mixture was 
diluted to 2,000 times, the original solution having a concentra- 
tion of 34.52 g. iron as FejOs and 20.00 g. chromium as Cr^Og per 
liter. This shows remarkably that ferric chloride is far more 
stable toward dilution than is ferric sulphate for the same 
basicity, and in this respect ferric chloride behaves in much the 



34 LEATHER CHEMISTS ASSOCIATION 

same way as chromic chloride or other chromic salts. The con- 
clusion to be drawn from this would be that in order to get a 
stable liquor as much of the ferric salt in the tan liquor as possible 
should be in the form of a chloride, but unfortunately the main 
supply of ferrous salt is already in the form of a sulphate ("cop- 
peras") owing to the cheapness and convenience of sulphuric acid 
for pickling purposes in the foundry and steel works. With the 
chlorine oxidation, however, a third of the acid radical is con- 
veniently secured in the form of a chloride. 



IRON tannage; 35 

Section V. On the Rei^ation of Basicity to Stability in 

Iron Liquor. 

The instability of an iron liquor, or rather, the ease with which 
hydrated ferric oxide separates out from a solution, depends 
upon the degree of acidity of the solution. The liquor used in 
tanning is normally more alkaline than that which corresponds to 
a neutral salt, e. g., Fe2( 804)3. The liquor, however, reacts acid 
even when it is constitutionally basic. If an alkali is introduced, 
the OH" ions from the alkali tend to precipitate ferric iron as 
ferric hydroxide or as some basic ferric compound, but the super- 
natant solution still reacts acid. Only after all the ferric iron 
has been precipitated, does the solution begin to react alkaline. 

It is evident that in order to study the stability of an iron 
liquor with regard to its basicity, it is necessary to know quanti- 
tatively the relation between the Fe+ + + ion in solution and the 
acid radical or radicals present. The subject presents some diffi- 
culty, as the acid radical in ferric sulphate solution may be com- 
posed of, besides the sulphate ion SO^, such other negative ions 
as NO3", Cl~, etc. This is not uncommon as the ferric sulphate 
in commerce is generally obtained by the oxidation of copperas 
with HNO3 and HgSO^, some of the HNO3 may remain in the 
ferric salt solution formed. In order to eliminate as much as 
possible complexities of this nature, there was chosen as the start- 
ing point a white powder of ferric sulphate as nearly chemically 
pure as possible. A solution of this salt (which dissolves very 
slowly in water) was made containing 133 g. of the air-dried 
powder to a liter. The solution was allowed to stand in a closed 
bottle for four weeks, when a small amount of precipitate col- 
lected at the bottom. The solution tested for CI" with AgNOg 
solution gave a negative result. It was then tested for NO3" by 
adding concentrated H2SO4 and then ferrous ammonium sulphate 
solution. No colored ring was observed. This test is not very 
delicate. With diphenylamine in HgSO^ solution a violet to blue 
coloration is observed, but as the solution contains ferric iron 
this test for NO3" in the presence of iron can not be regarded as 
conclusive.-^ The test was therefore further elaborated by dis- 
tilling with ferrous sulphate and H2SO4 and receiving the dis- 
tillate with a 50 cc. 3 per cent. NaOH solution contained in a 250 
^ See Tradwell-Hall, Analytical Chemistry, Vol. I, p. 394 (1916). 



36 i,e;athi;r chemists association 

cc. Erlenmeyer flask. The distillate was acidified and shaken with 
5 cc. of chloroform after adding 5 cc. of 10 per cent. KI solu- 
tion.2« The test gave a negative result for NO3. After obtaining 
conclusive negative tests both for CI" and NOg", it was then neces- 
sary to determine the quantitative relationships between the ferric 
iron and the sulphate radical in the solution. For this purpose a 
50 cc. portion of this ferric sulphate solution was diluted to 500 
cc,, 25 cc. of which were taken for each of the analyses described. 
In order to establish the quantitative relationships accurately it 
was considered advisable to determine the iron and the acid rad- 
ical (SO^^") each by two independent methods. Iron was deter- 
mined gravimetrically by precipitating with NH^OH in the pres- 
ence of NH4CI, and independently again by the Zimmermann- 
Reinhardt volumetric method. The SO/ radical was determined 
gravimetrically by precipitating with BaClg in HCl solution and 
independently again by the acidity determination by titrating in 
the heat against N/io NaOH, using i cc. Yz per cent, phenol- 
phthalein solution as an indicator. In each analysis at least two 
portions were carried and the results checked. The results were 
tabulated as follows : 

TABLE II.— Results of Analyses for Fe+++ and S04== 
BY Independent Methods. 

No. of equivalents con- 
tained in the 25 cc. 
Method of determination (diL solution) 

Fe+ + + Zimmermann-Reirihardt 0.003853 

Gravimetric as FcjOa 0.003866 

S04= Gravimetric as BaS04 0.003861 

Titration against N/io NaOH 0.003872 

Note. i. — The Zimmermann-Reinhardt method is not suited for H2SO4 
solution because (i) the ferric sulphate solution in the presence of 
H2SO4 has a less pronounced j^ellow color to guide the reduction by 
SnCU, (2) the reduction by SnCU is much slower in the H2SO4 solu- 
tion than in the HCl solution, and (3) in the H2SO4 solution a pre- 
cipitate is more Hable to form in the solution during the reduction 
unless a large excess of the H2SO4 is present. H3PO4 alone without 
MnS04 and H2SO4 was used. The result was found to be slightly 
affected by the amount of excess of SnCU employed. 

Note 2.— In the acid determination by titration with NaOH, the difference 
between the end points in the cold and in the heat is not great, being 
about I per cent, of the total burette reading in the case of ferric 
sulphate solution and i^/^ to 2 per cent, in the case of ferric chloride 
solution. It is remarkable to note that the corresponding difference 

°°See A. A. Noyes, Quantitative Chemical Analysis, (1915) p. 113. 



IRON tannage; 37 

in the case of chromic chloride solution is as much as lo per cent, of 
the total burette reading. 

By "No. of Equivalents" is meant the number of equivalent 
weights of Fe+ + + and SO/ contained in the above 25 cc. of 
diluted ferric sulphate solution, so that if the solid ferric sulphate 
from which the solution was made is chemically pure, i. e., con- 
tains nothing but Fe2( 804)3 and water, the number of equivalent 
weights of Fe+++ (which represents 3 equivalents per formal 
weight) should be exactly equal to that of 80^= (which repre- 
sents 2 equivalents per formal weight). The ratios of the number 
of equivalent weights of Fe+++ and SO^^ in the solution as de- 
termined are as follows : 

TABLE III. — Ratios of Equivalents or Fe+ + + to Equivalents oe 
S04= IN Ferric Sulphate Solution. 

S04^ by gravimetric SO4 ~~ by titration 

Fe+ + + by Zimmermann-Reinhardt i.ooo : 1.002 i.ooo : 1.005 

Fe+ + + by Gravimetric i.ooo : 0.998 i.ooo : 1.002 

The closeness with which the above results agree indicates that 
the ferric sulphate employed is substantially chemically pure. 
Knowing the exact constitution of the ferric salt solution it was 
then necessary to study the stability of the ferric salt in solution 
by changing the degree of basicity. This was done by adding a 
calculated amount of NagCOg solution of a known strength to a 
given quantity of Fe2( 80^)3 solution and finally making up to the 
same total volume with distilled water in each case. In this study 
there was employed a strength of the ferric salt solution in each 
case not far from that of the iron liquor used in the actual drum 
tanning operation. This was estimated to be from 2 to 4 per 
cent, iron calculated as Fe2( 804)3. In this connection it might 
be added that NaaCOg was used rather than NaOH, as in actual 
practice in the tannery sal soda or sodium bicarbonate is generally 
used for such a purpose. A 0.5000 N NagCOg solution was made 
from a thoroughly dried, anhydrous C. P. sodium carbonate. For 
study, a 25 cc. portion of the above ferric sulphate solution con- 
taining 10.26 g. Fe2( 804)3 per 100 cc. was taken, a calculated 
quantity of this Na2C03 solution run in from a burette, and the 
total volume made up to 65 cc. with distilled water. The results 
were tabulated in Table IV. Although during the introduction of 



38 IvElATHER CHEMISTS ASSOCIATION 

NaoCOg there was a brisk evolution of COg gas, there remained 
in the suspension some COg^ ions in addition to the OH" ions, 
so that the precipitate formed was in the nature of a basic ferric 
carbonate or a mixture of ferric hydroxide and ferric carbonate.* 
It was noted that the precipitate separated out from No. 7 
more readily than it did from Nos. 8, 9, 10, and 11. It is clear 
that the ferric sulphate solution having a basicity higher than that 
which corresponds to No. 6 is unstable and a yellow precipitate of 
FcaOs.xHgO or some basic ferric carbonate soon separates out. 
It was found from the tanning experiments also that this degree 
of basicity in the case of a sulphate liquor was as high as could 
safely be employed without danger of causing a yellow precipitate 
to separate out on short standing and also that if the iron liquor 
employed was of a higher basicity than this, it would rapidly im- 
part a yellow color to the surface of the pelt after drumming from 
15 to 30 minutes. 

Because of the purely scientific interest involved NaOH was 
also used in place of NaaCOg so that in this case only OH" ions 
were introduced and the complication of having some COg^ ions 
present in the solutions was eliminated. The results are shown in 
Table V. Comparing Table IV and Table V, it is remarkable to 
note how the two series run parallel to each other. Table V like 
Table IV also indicates that in the case of the sulphate liquor Nos. 
6 and 7 represent the highest basicity beyond which the iron liquor 
yields much precipitate. The difference between the two series 
is that in the case of NagCOg any precipitate first formed can be 
caused to disappear on continued shaking to a thick solution from 
which the final precipitate appears after a lapse of from 12 to 30 
minutes thereafter while the precipitate from the NaOH solu- 
tions is immediate and persistent. The color of the solutions in 
Table V is also somewhat deeper. The report that the best iron 
liquor to be employed for tanning is that which possesses a basicity 
corresponding to the formula Fe(0H)S04 is not borne out by 
this study or by the tanning experiments. 

There are other evidences that independently show that 

* To determine how much C03= is present in the suspension, CO2 was 
distilled from it after the precipitate had just separated out, with excess 
of H2SO4 and received in a known NaOH solution. Titration gave the 
amount of C03= present in the suspension to be 63.6 per cent, of the total 
added. 



IRON TANNAGE 39 

Fe(0H)S04, or a basic ferric sulphate corresponding to this 
basicity, is very unstable and rapidly yields a precipitate from its 
solution. For instance, when a ferrous sulphate solution is oxi- 
dized by hydrogen peroxide without the addition of any acid, a 
precipitate is soon formed in the solution. This basic ferric sul- 
phate is formed by the following reaction : 

2FeSO, + H2O2 — ► 2Fe(OH)S04. 
Other neutral oxidizing agents, such as KBrOg, that can efifect 
this oxidation without the addition of an acid to the neutral fer- 
rous sulphate solution, produce similar precipitates in the result- 
ing ferric salt solution. 

In the case of ferric chloride solution, the difference is aston- 
ishing. A similar series of the ferric chloride solution was 
studied using NagCOg solution. The results are tabulated in 
Table VI. It is remarkable that throughout the whole series 
studied no precipitate was formed after one week's standing even 
where the basicity ratio was higher than the highest in the sulphate 
series studied above. This shows indeed that the ferric chloride 
is far more stable in a basic solution than the corresponding sul- 
phate. After two weeks' standing, however, precipitate began to 
appear in solutions Nos. 3, 4, 5, and 6. It is interesting to note 
that here as in the sulphate series above (Tables IV and V) the 
precipitate separated out more readily from solutions of lower 
basicity in the series (Nos. 3, 4, 5, and 6) than form those at the 
end having high basicities, such as Nos. 10, 11, or 12. 

The "acidity" determination, according to the method of 
Thomas and Baldwin-^ for the solutions of various basicity in 
the series studied above, can not be made in the case of the ferric 
salt solution, because it is found that ferric iron is rapidly reduced 
by the hydrogen in the presence of the platinum electrode, and no 
reading can be obtained. 

The determination of the hydrogen ion concentration ("acid- 
ity") in iron tan liquor by a study of the rate of hydrolysis of 
sucrose has also a similar difficulty as one of the products of 
hydrolysis (glucose) has a reducing action on the ferric iron in 
the hot solution. 

" "The Acidity of Chrome Liquors," by A. W. Thomas and M. E. 
Baldwin, This Jour., May, 1918, p. 192. 



40 



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I^EATHER CHEMISTS ASSOCIATION 



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IRON TANNAGE 43 

Section VI. Behavior of the Pelt Towards 
Iron Tan Liquor. 

In view of the report-^ that iron tan Hquor has the same acidity 
at the end of the tanning operation as it had at the beginning and 
also of a proposed process of iron tannage^® using the iron tan 
liquor over and over again in a cycle without mentioning the 
necessity of readjusting the acidity of the liquor it was thought 
advisable to study this matter. Pickled sheepskin was cut into 
rectangular pieces of about 4 inches by 5 inches. These pieces 
were placed in tepid water for a short time and, when softened, 
introduced into cold water containing a small amount of salt to 
prevent plumping. The skin was then carefully neutralized with 
NaaCOg until all mineral acid was removed, using methyl orange 
as an indicator. The skin was rinsed off and the excess water 
squeezed out so that it was roughly in the same condition as 
pickled skins that have been horsed up over night. The reason 
for neutralizing the skin in our experiments was to avoid intro- 
ducing into the tan liquor an indefinite amount of the mineral 
acid present in the pickled skin. The tan liquor used in these 
experiments was a basic ferric sulphate or chloride solution, hav- 
ing in general a basicity of 3 equivalents of mineral acid radical 
to 4 equivalents of Fe+ + + and containing iron from ii^^ to 16^ 
g. FcgOs per liter. The volume of the tan liquor in cc. equaled 
from 3 to 4 times the weight in grams of the skin with excess 
water pressed out. Tanning was carried out in a glass jar of i^/g 
liters capacity set in a bottle shaker making about 35 R. P. M. 
The actual weight of the skin in these experiments was from 150 
to 200 grams, and the total volume of the tan liquor 600 cc. The 
amount of iron used (calculated as FegOg, therefore, varied from 
4.0 to 6.0 per cent, of the weight of the pelt in the thoroughly 
drained condition. Both NagCOg and NaOH were employed to 
bring about the proper basicity in the liquor for tanning. In the 
case of Na2C03, COg gas continued to be given off during tan- 
ning. 

The detailed procedure was as follows. A desired amount of 
the stock ferric solution of known concentration and acidity was 

^ "Notes sur le Tannage aux Sels de Fer," by V. Casaburi, Le Cuir, 
August I, 1919. 

^ Bystron and Vietinghofif's Patent (German Pats. Nos. 255,320 et seq.). 



44 i.e;athe;r chemists association 

taken, and a calculated quantity of NaOH or NagCOg solution of 
known strength added in order to obtain the desired basicity for 
tanning. The solution was then diluted according to the above 
volume relation. The skin was immediately placed in the liquor 
and the container shaken in the bottle shaker. 5 cc. samples were 
taken for analysis at an interval of 15 minutes or longer. Iron 
was determined by the Zimmermann-Reinhardt method and the 
acidity (H2SO4 or HCl) by NaOH titration using i cc. >4 per 
cent, phenolphthalein solution as an indicator.* The acid deter- 
mination was obtained by titration with N/io NaOH first in the 
cold; and after the end-point has been reached, the solution was 
brought to just below boiling and titration continued until the end 
point was again reached. The difference between the cold and the 
hot end-points was only 0.15 to 0.35 cc. N/io NaOH for a total 
burette reading of 10 — 30 cc. Four independent experiments 
were carried out in the case of the ferric sulphate liquor, but 
one set of data and results from one of these experiments will 
be given here which may be considered as typical. 

Ferric Sulphate Tan Liquor. 
Data : 

Sheepskin (with excess water pressed out) ' 167 g. 

Ferric sulphate liquor — 

(a) Volume for tanning ; 600 cc. 

(b) Concentration (calculated as Fe203) .... 16.33 g- per 1. 

(S04= Equiv.) 

(c) Ratio of 0.742 

(Fe+ + + Equiv.) 

Salt (about 5^^ per cent.) 9 g. 

* Mineral acid and iron can be determined in the same sample by first 
titrating with NaOH solution in the hot, allowing ferric hydroxide to 
collect at the bottom, filtering off the precipitate, dissolving it from the 
filter with a hot, dilute HCl, and then determining the iron in solution by 
the Zimmermann-Reinhardt method. 

The results are tabulated in Table VH. The skin at the end 
of each experiment was well tanned save for the neutralization 
operation which would be required in actual practice. 



IRON tannage; 



45 



TABLE VII. — Behavior of Neutral Pelt Towards Basic 
Ferric Sulphate Liquor. 



Sample 
No. 


Time interval 

between which 

.samples were 

taken 


Cc. NaOH 
in the hot 

N = 0.1096 


Equiv. 
H..,S04 
per 1. 


Cc. KMn04 

N= 0.1004 


Rquiv. 
iron 
per 1. 


Ratio of Equiv. 
of 

SO+=toFe+ + + 


I* 






20.72 


0-455 


10.17 


0.613 


742 : 


1000 






2 


IS 


min. 


17.88 


0.392 


8.63 


0.520 


754 • 


1000 


3 


IS 


min. 


16.80 


0.368 


7-51 


0.452 


815: 


1000 


4 


15 


min. 


16.13 


0-354 


7.21 


0.435 


814 : 


1000 


5 


IS 


min. 


15-86 


0.348 


6.80 


0.410 


849 : 


1000 


6 


IS 


min. 


15-54 


0.341 


6.61 


0.398 


858: 


1000 


7 


IS 


min. 


15-30 


0.336 


6.4s 


0.388 


866 : 


1000 


8 


IS 


min. 


15-15 


0.332 


6.38 


0.38s 


862 : 


1000 


9 


IS 


min. 


15.10 


0.331 


6.34 


0.382 


867 : 


1000 


ID 
II 


30 

24 


min. 
hrs. 


14.87 


0.326 


6.28 


0.378 


862 : 


1000 




Continuous 
















shaking 


14-51 


0.318 


6.17 


0.372 


855 : 


1000 



*NoTE. — The sample of this basic ferric sulphate tan liquor yielded a pre- 
cipitate on standing, but after the skin was placed in it and tanned, 
all subsequent samples no longer yielded any precipitate. During 
tanning, the color of the liquor became lighter- — from deep red to 
yellow — and the skin, on the other hand, was gradually colored red. 

An additional experiment was carried out using a ferric chlo- 
ride liquor in place of the ferric sulphate liquor used above. In 
the following are tabulated the data of this experiment. 



Ferric Chloride Tan Liquor. 
Data: 

Sheepskin (with excess water pressed out) 152 g. 

Ferric chloride liquor — • 

(a) Volume for tanning 600 cc. 

(b) Concentration (calculated as FcsOs) . . . . 11.60 g. per 1. 

(CI- Equiv.) 

(c) Ratio of 0.765 

(Fe+ + + Equiv.) 
Salt (about 5 per cent.) 8 g. 

The results are tabulated in Table VIII. As in the case of the 
experiments using ferric sulphate liquor the skin was also well 
tanned in the ferric chloride liquor and no neutralization was 
effected. 



46 



LEATHER CHEMISTS ASSOCIATION 





TABLE VIII 


. — Behavior ot Neutral Pelt Towards Basic 








Ferric Chloride Liquor. 










Time interval 

between which 

samples were 

taken 


HCl 






Fe++ + 


Ratio of 
of 

Cl~ to F 




Sample 
No. 


Cc. NaOH 
in the hot 
N = 0.1 104 


Equiv. 
HCl 
per 1. 


Cc. 

N 


KMnOi 
= 0.1087 


Equiv. 
iron 
per 1. 


Equiv. 


I* 




15.00 


0.333 




6.66 


0.435 


765 : 


1000 






2 


15 min. 


I316 


0.291 




574 


0.374 


778: 


1000 


3 


15 min. 


12.44 


0.27S 




531 


0.346 


795 : 


1000 


4 


15 min. 


12.07 


0.267 




5-07 


0.331 


806 : 


1000 


5 


15 min. 


11-95 


0.264 




4.76 


0.3 1 1 


849 : 


1000 


6 


15 min. 


11.90 


0.263 




4-63 


0.302 


871 : 


1000 


7 


15 min. 


11.89 


0.263 




4-59 


0.300 


876 : 


1000 


8 


15 min. 


11.82 


0.261 




4.61 


0.301 


868 : 


1000 


9 


15 min. 


11.80 


0.261 




4-51 


0.294 


887 : 


1000 


10 


24 hrs. 
Continuous 


















shaking 


11.70 


0.259 




4-47 


0.292 


887 : 


1000 



*NoTE. — None of these samples (Including sample No. i) of this basic 
ferric chloride liquor yielded any precipitate on standing. The colors 
of these samples were decreased from deep red (sample No. i) to 
light yellow (sample No. 10). On the other hand, the red color of 
the skins became deepened as the tanning operation progressed. 

From these results the following important conclusions can be 
drawn : 

(i) Iron is taken up by the skin very rapidly at the beginning 
of the tanning operation, and from 30 to 40 per cent, of the 
total is taken up by the pelt before neutralization. 

(2) The mineral acid (sulphuric acid or hydrochloric acid) is 
also taken up by the skin in a similar manner, the total amount 
absorbed by the neutral pelt being in general about 20 to 30 per 
cent, of the total. (If pickled skins are not neutralized before 
tanning as in actual practice, a correspondingly less amount of 
the acid will be taken up by the skin. That the skin absorbs 
the sulphuric or hydrochloric acid from the liquor is corroborated 
in actual tanning practice by the fact that only 70 — So per cent, 
of the theoretical amount of alkali is required to eifect the com- 
plete neutralization. See later Section on Pure Iron Tannage.) 

(3) Although the mineral acid is also taken up by the pelt, it 
is not taken up in the same proportion as the iron so that the 
liquor is more acid towards the end of the tanning operation than 
at the beginning. 



IRON TANNAGE 47 

(4) The curves of absorption of both the iron and the acid by 
the pelt with respect to the time of tanning approach some con- 
stant horizontal lines asymptotically. 

(5) The tanning reaction is practically completed within one 
and a half hours of drumming and the neutralization operation 
can begin after 1^4 to lYz hours of drumming, it being neither 
necessary nor advisable tO allow the pelt to remain in the liquor 
for longer than i^^ hours before neutralization takes place. 

(6) Both the basic ferric sulphate liquor and the basic ferric 
chloride liquor behave alike towards the pelt, the only difference 
being that the ferric chloride liquor possesses decidedly less 
tendency to yield the precipitate of hydrated ferric oxide than 
does the ferric sulphate liquor. 

Although burette readings for NaOH are expressed in four 
significant figures, the precision for the results obtained in many 
cases is probably not much better than i per cent. For, in the 
first place, the samples taken for analysis are small (only 5 cc.) 
and, in the second place, the organic particles, such as fatty mat- 
ters, skin fibers, etc., present in the tan liquor make accurate 
sampling rather difficult. Furthermore, the presence of grease 
causes tiny drops of the liquor to adhere to the walls of the 
pipette, so that frequent cleaning by means of a cleaning solution 
is necessary. Considerable difficulty is also experienced due to 
the fact that the presence of the organic matter in the liquor inter- 
feres with the determination of iron by the KMnO^ titration. 
This difficulty is finally overcome by oxidizing all the organic 
matter present in the sample with KMnO^ in the presence of HCl 
until a purple color is seen. The sample is then heated, reduced 
with SnCla, and titrated with KMnO^ solution in the usual 
manner. 

It was observed in tanning experiments using a wooden drum 
that a completely oxidized iron tan liquor gave a copious preci- 
pitate when tested with K3Fe(CN)6 solution at the end of the 
tanning operation. Evidently the tan liquor is subject to reduc- 
tion by the skin, the woody material and the metallic parts of the 
drum, and organic impurities that may collect in the drum. The 
amount of reduction in the tan liquor by the skin alone has been 
determined. For such purpose, the skin was tanned in a glass 
container with a completely oxidized ferric sulphate liquor. The 



48 



i.e;athe;r chemists association 



f/c. -J// 



Curves Showing Absorption of Fe and H2SO4 during Tanning 



0.6 



• — , 



0.5 



0.4 



0.3 



0.2 



I 
I 



OJ 



<J 



\ 


» 






















\ 


1^" 


1^ 


►.^ ■ 


r>= 
























-; 






' — °— -ji^ — 1 i 1 








^c/C Concf>ni-n^1-}nn 


/'H .^P ■"! 




















(f'sou^) 























































0-I5 0:30 0:45 m 1:15 1:30 1:45 2:00 2:15 2:30 

Time of Tanning in Hours and Minutes ' 



24:00 



+ 05 



.5 0.7 

I 

.Q 0.6 



r/c.-iv 

Curve Showing Increase in Acid Concentration during Tanning 
For Ferric Sulphate Liquor 



C) 



Q^ as^ 



>!_• 



0:15 0:30 0:45 1:00 /:I5 1:30 1:45 2:00 2:15 2:30 24W 

Time of Tanning in Hours and Minutes 



IRON TANNAGE 



49 



Curves Shoi/ving Msorption ofFe and HCl during Tanning 



0.6 
Cj 0.6 




























V 


























1 < 




^*^( 


1*^^^^ 


i_ 


>— i.1 




/■^^^^ 










\ 


kl 


''***"n^^ 1 1 1 








1 - 








Acid Concentr 


ation 


(HCi) 










i, 
























































? 


•/5 


30 


■45 /■ 


^^ /• 


/5 /•• 


30 1- 


45 2 


00 






24^ 



Time of Tann'mg in hours and Minutes 



r/o-v/ 
Curire Showing Increase in Acid Concentration 
in Fe Cla Tan Liquor during Tanning 




aJs 0-30 d-45 Iw tB Wo ms IW 

Time of Tanning in hours and Minutes 



50 ivEATHER che;mists association 

skin was allowed to remain in the liquor with continuous shaking 
for 24 hours. In this particular experiment, an amount of iron 
containing 14^ per cent. Fe2( 804)3 of the weight of the drained 
skin, dissolved in water equal to 3^ times the weight of the skin, 
was used. Ferric iron in the sample taken at the end of 24 hours 
was determined by means of a titanous sulphate, TiaC 804)3 in 
H28O4 solution using ^ cc. normal NH4CNS solution as an 
indicator, while total iron was determined by KMn04 titration 
after oxidizing off all the organic impurities in the sample with 
KMn04. It was found that 4.4 per cent, of the total iron in the 
liquor had been reduced to the ferrous state. This amount of 
reduction was exclusively due to the skin. In actual practice 
where a wooden drum is used with whatever dirt that may collect 
in it, the percentage of the iron reduced must be greater. 



IRON tannage; 51 

Section VII. Experiments on Tanning With 
Ferric Hydroxide Hydrosol. 

A ferric hydroxide hydrosol was prepared and applied to tan- 
ning for two reasons : ( i ) that it was desired to study the tanning 
action of ferric iron completely in the colloid state, and (2) that 
as a sol requires for its stability only the small amount of an 
electrolyte that is retained with the colloid,* the tan liquor can be 
considered to be practically free from an electrolyte and conse- 
quently the function of a neutral electrolyte such as NaCl, 
NagSO^, etc., used in a mineral tanning liquor will thereby be 
disclosed. The sol was made by the peptization method. For 
such a purpose a fairly concentrated (about 30 per cent.) ferric 
chloride solution was taken and a dilute ammonia solution ( i vol. 
0.90 NH4OH to 8 or 10 vol. water) added drop by drop, while the 
solution was stirred by a mechanical stirrer. On adding NH^OH 
the yellow-red solution turned brown and finally almost black by 
reflected light. The addition of NH3 was continued until solid 
particles of ferric hydroxide were re-dissolved only after 5 to 10 
minutes continuous stirring. The sol was dialyzed in collodion 
sacks first in running tap water over night and then in distilled 
water for 6 days, changing the distilled water three times every 
24 hours. The sol thus obtained was rather diluted as much water 
had entered the sacks during dialysis by virtue of the osmotic 
pressure, and it was necessary to concentrate it by evaporation. 
After concentration it was filtered through cotton. 

This sol was coagulated by an electrolyte such as NaCl when a 
certain concentration of it was reached. On standing the coagu- 
lated ferric hydroxide settled at the bottom, leaving the super- 
natant solution colorless. When it was coagulated by HCl, added 
in some excess, however, the precipitate, on standing, re-dissolved 
giving a yellow solution of ferric chloride. A dilute sol prepared 
as above but dialyzed for seven days showed a misty beam of light 
under an ultramicroscope. The particles are so small that they 
can not be counted and their motions are not discernible although 
a distinct mist is seen. The electrical charge of the sol was de- 
termined by cataphoresis. This was determined in the ordinary 
U-tubes using KCl solution of the same conductivity as the sol in 

* The "Complex" Theory of Colloids. 



52 i,eathe;r che;mists association 

the upper portions of each branch in which the electrode was im- 
mersed. The sol moved toward the negative electrode, showing it 
to be positively charged, as would have been expected. 

The tanning action of the sol toward a carefully neutralized, 
nearly salt-free sheepskin was studied. For this purpose, a piece 
of pickled sheepskin (4^ in. x 8^4 in.) was first softened in 
water in the presence of a small amount of salt. It was carefully 
neutralized with NagCOg solution using methyl orange as an in- 
dicator, and the salt was washed out from the skin as far as pos- 
sible (without causing undue plumping) when only a small 
amount of AgCl was formed in the wash water tested with an 
AgNOs solution. The excess water was pressed out from the 
skin and its weight in this condition was 51 g. A 250 cc. portion 
of the ferric hydroxide hydrosol (analysis giving 0.963 g. FeaOj 
per 100 cc.) was taken and the neutralized, nearly salt-free skin 
shaken in it continuously for 2^ hours. The sol was not coagu- 
lated but the skin swelled to about three times its original thick- 
ness, becoming stiff and rubber-like. It was not tanned. Five 
per cent, salt (2.5 g.) was next added and the shaking continued 
for one hour longer. The skin had fallen but was not tanned and 
the inner layer was not even penetrated. The sol was coagulated 
by the addition of the sodium chloride. 

The experiment was repeated in exactly the same manner, us- 
ing, however, a well pickled skin instead of the de-pickled and 
salt-free skin. No salt was added. The hydrosol was again 
coagulated by the small amount of the H2SO4 and NaCl left in 
the pickled skin, yet the skin swelled badly. The cross section of 
the skin showed red lines on the outside edges and a large white 
untanned band in between. 

From this it appears that the presence of an electrolyte such as 
NaCl or NajSO^ in the tan liquor is to prevent plumping so that 
the pelt is kept "fallen" in the liquor so as to be readily pene- 
trated by the tanning agent. It must be observed that the plump- 
ing of the pelt in the hydrosol is not caused by the acid liberated 
during tanning, for there is only a negligible amount of an acid 
radical that can be retained in the sol, but by simple inhibition of 
water by the gelatine material of the pelt in this neutral sol. It 
appears also that ferric hydroxide in a pure sol form can not be 
employed for tanning purposes; for without a sufficient amount 



IRON tannage; 53 

of an electrolyte, the pelt would be caused to swell and not be 
penetrated ; but if sufficient electrolyte is added to prevent plump- 
ing, the sol is coagulated at once to a gel which likewise would 
not penetrate the pelt. It is evident that a pure iron sol will not 
act as a tanning agent ; hence the theory of leather formation by 
colloidal co-precipitation alone is open to doubt, and the reaction 
may be purely chemical. 



54 IvDATHER CHl^MISTS ASSOCIATION 

Section VIII. Generai, Experimentai, Work on Iron 
Tanning. 

The requirements of a good tannage are (i) that the pelt shall 
be converted into a net work of isolated fibers and become no 
longer putrescible, and (2) that this conversion shall be irrever- 
sible, i. e., the leather so obtained shall not be readily affected, or 
reverted to the raw condition, by water or other agency (acid- 
alkali treatments excepted). In addition it might be mentioned 
that the leather obtained shall keep well, i. e., it shall not spon- 
taneously deteriorate on storing or in use within a reasonable 
length of period. Iron tannage can yield a leather that will ful- 
fill all these requirements when it is properly carried out. 

In research work on this subject many difficulties have been 
encountered. Since no detailed data on the subject are available 
in the existing literature, much time has been spent in determining 
conditions for a successful working. A ferrous sulphate solution 
was employed as such and found to have no tanning property.* 
Oxidation of the iron was then resorted to, but the question arose 
as to what oxidizing agent should be employed and in what man- 
ner the oxidation should be effected. Sodium nitrite and sul- 
phuric acid, bleaching powder and acetic acid or sulphuric acid, 
sodium dichromate and sulphuric acid were among the oxidizing 
agents first employed. The question of completeness in oxidation, 
the relation of acidity of ferrous sulphate solution to oxidation, 
and the reduction of the ferric iron in the liquor by organic mate- 
rials in contact with it were matters that were gradually brought 
to light. The basicity of the iron liquor used for tanning exercised 

* The experiment was carried out as follows : 

Skin (sheepskin pickled, with water pressed out).... 40 g. 

Salt (5 per cent.) 2 g. 

Ferrous sulphate (FeS04.7H20) (14 per cent.) 5.6 g. 

Water (3 times the weight of skin) 120 cc. 

The solution was then made slightly alkaline up to the point of the 
beginning of the precipitation of the green Fe(0H)2. The skin was 
shaken in it for i^ hours and found to remain soft and unaffected. On 
drying the skin so treated was practically in the same condition as a dry 
pickled skin with dark spots and areas of contracted grain shown on the 
grain side. It was affected by water and plumped in it 



IRON TANNAGE 



55 



an unlooked-for influence.** For, when it was not properly ad- 
justed, either no satisfactory tannage was obtained, or much yel- 
low precipitate was deposited so that the pelt was difficult to tan 
through. 

After some experimentation a tannage was found possible and 
the pelt was apparently well tanned while in wet condition. On 
drying, however, the pelt shrunk and the color of the grain was 
uneven. Dark wrinkled spots appeared on the grain and often 
the skin on drying looked horny and transparent. The leather 
was stiff, and the grain brittle.f 

Since a comparatively high percentage of the iron salt of the 
weight of the pelt is needed, this necessitates the use of a small 
amount of water and the employment of a somewhat concentrated 
liquor ("short liquor") in an attempt to cut down the quantity of 



* *To investigate the effect of the basicity of the tan liquor on the skin 
(neutraUzed) tanned in it, four tan Hquors with different degrees of 
basicity were prepared and the skins tanned in them. The skins, after 
being shaken in these liquors for 15 minutes, were examined and the 
results tabulated as follows : 







Table IX. 




Basicity ratio 




Color of skin 


Character of 


Equiv. Fe +++ 


Color of 


shaken for 


skin after 


Bquiv. SO4 = 


tan liquor 


15 min. 


15 min. shaking 


6/S 


Deep red 


Reddish straw 


Skin penetrated by 




(Clear sol.) 


color 


sol. 


5/4 


Dark red 
(Clear sol.) 


Light red 


Skin tanned through 


4/3 


Dark red 
(Cloudy sol.) 


Red-yellow 


Skin tanned through 


3/2 


Black red 


Brown-yellow 


Pigment crust on 




Turbid sus. 




surface layer 
Skin barely tanned 
through 



It will be noticed that when the liquor is of the right basicity the color 
of the skin in contact with it should be more red than yellow. 



t If, however, before the pelt (skin) was permitted to dry completely 
it was stretched or knee-slaked while still in a sammied condition, the 
brittleness of the grain on complete drying could be overcome. This 
amounts to separating the fibers by mechanical means rather than from 
the natural result of tanning. A well pickled skin could be made soft and 
flexible as if tanned, when worked in the same way. 



56 i,e;athe;r chemists association 

the iron salt taken.* The phenomenon of "grain drawing" is 
then apt to occur if the liquor is not carefully introduced and the 
pelt properly prepared. 

Finally, after satisfactory tannage could be obtained the prob- 
lem of coloring presented another phase of difficulty. The yellow- 
red color of the leather makes dyeing to a light color ("fancy 
color") impossible. Further, the iron in the leather is active so 
that it combines with many substances to form insoluble com- 
pounds having generally an objectionable color. Vegetable re- 
tanning is limited to cases where a gray or black leather is desired. 
The interference of iron with dyeing by means of basic dyes to 
a color other than black constitutes another difficulty on account 
of the chemical action of iron on many a vegetable mordant re- 
quired when such dyes are to be used. On the other hand, in 
certain cases where chemical activity of iron is utilized for the 
coloring of the leather by a treatment with substances capable of 
producing color lakes with iron (e. g., with K4Fe(CN)6 for blue 
coloring) the leather then becomes hard and brittle, probably due 
to the withdrawal of iron from the fibers for the formation of 
the inert color lakes. Consequently the leather is to a greater or 
less extent detanned.f Furthermore, the color of the leather thus 
produced is not fast and is slowly washed out unless this treat- 
ment is immediately followed by oiling or fat-liquoring. On the 
other hand, the same detanning effect also results when -an attempt 
is made to bleach the iron tanned leather by means of a reducing 
agent such as bisulphite or thiosulphate followed by an acid. 

Some methods that have been employed to overcome these diffi- 
culties may be mentioned. It was found that improper tannage 
more than anything else was responsible for the brittleness of 
the leather. When a pelt is not uniformly tanned through, due to 
either the liquor employed being too alkaline or subsequent neu- 

* Similar improvement has been successfully made in chrome tannage. 
Thu^, from a private communication, a process of two-bath chrome tannage 
for skins consisting of (i) pickling with 3 per cent, salt, i per cent. 66° 
Be. H2SO4 in 2 gallons water per 100 pounds, drumming for 20 minutes; 
(2) chroming with 3 per cent. Na2Cr207.2H20, i per cent. 66° Be. H2SO4 
in 2 gallons water, drumming for 2 hours; and (3) reducing with ij^ per 
cent. 66° Be. H2SO4 in i gallon water (added first) and 14^ per cent. 
"hypo" in 2 gallons of water, drumming for 2 hours, has been successfully 
employed, giving a very soft, light-colored leather. 

t If the neutralized tanned pelt is first dried to "crust" and then wetted 
back for this treatment, the effect of detanning is less marked. 



IRON TANNAGE 57 

tralization too rapid, the outer layer (the grain) becomes dense 
and crusty, while the inner layer remains soft. The whole pelt 
on drying, therefore does not contract uniformly and the shrink- 
ing or curling up of the pelt results. This leads to. the breaking 
of the grain on bending. When much iron in the tan liquor has 
been reduced to the ferrous state and found its way to the pelt, 
it v/ill become oxidized during drying. This appears to be respon- 
sible for dark, wrinkled, hard spots appearing on the grain. 

The solutions to some of these problems were found in the use 
of a completely oxidized iron tan liquor; the employment of a 
small excess of the oxidizing agent in the liquor; the maintenance 
of iron in the ferric state by means of an after oxidation ; the use 
of optimum basicity for tanning; and the careful neutralization 
of the pelt after tanning. All these will be dealt with at some 
length in the following sections. 

The ferric sulphate in solution is unstable and liable to be de- 
composed by hydrolysis from a neutral or slightly alkaline solu- 
tion, and is very rapidly precipitated upon the introduction of an 
alkali. Some attention has been devoted to investigating the pos- 
sibility of correcting this tendency. The use of organic protective 
colloids, or of gums that form mucilages in water, or of sub- 
stances that chemically combine with iron to prevent precipitation 
of ferric hydroxide from an alkaline solution, entails many com- 
plications. The difficulties in such cases are ( i ) that those nitro- 
genous protective colloids such as gelatine, egg albumin, blood 
albumin, etc., that are extensively used in connection with the 
other parts of leather manufacture are themselves coagulated by 
the highly concentrated tan liquor; (2) that the poly-hydroxy 
alcohols* in the form of syrup glucose, gum dextrin, starch, etc., 
exert a more or less reducing action on the ferric iron in the 
liquor, and (3) that compounds like Rochelle salt that hold up 
the ferric iron in an alkaline solution yield no tannage.f Other 
gummy bodies such as Irish moss, gum arable, gum tragacanth, 
etc., have hardly any effect. The presence of a chromic salt or 
an aluminum salt in the iron liquor yielding the corresponding 
hydroxide in an alkaline solution has some tendency to hold up 

* Glycerine can be used and seems to yield a tannage giving a soft, 
red, transparent leather. 

fThis speaks strongly of the chemical theory of leather formation in 
iron tannage. 



58 i,e;athe;r chemists association 

the precipitation of iron as ferric hydroxide and thus stabilizes it, 
especially when the amount present is equal to or greater than 
that of the iron, but this works best in a solution so alkaline as 
to peptize the chromic hydroxide or aluminium hydroxide.^" To 
regulate the speed of precipitation and also of the tanning action 
of iron in the pelt there is, at present, no satisfactory way except 
by the careful adjustment of the basicity of the iron liquor and 
of the control in subsequent neutralization. 

To minimize the interference of iron in the leather with dyeing, 
it is found that if the neutralized tanned pelt is first dried to 
"crust" before coloring, the iron appears better fixed in the fiber 
and its chemical activity greatly lessened. The use of pyrogallol 
tannins such as sumac, oak, etc., or certain less astringent catechol 
tannins such as mimosa, gambier, etc., then gives only a light 
grayish color so that these tannins can be used for mordanting 
as can also other vegetable matter such as fustic, etc., that do not 
produce a decided black with iron when used in small quantities. 
To keep the leather soft and flexible it is generally advisable to 
apply a somewhat heavy fat liquoring, or an oil re-tan using 
marine oils such as cod liver oil, shark liver oil, etc. 



'°Cf. "Hydrous Chromic Oxide" by C. F. Nagel, Jr., Jour. Phys. 
Chem., Vol. 19, p. 331 (1915). 

Also "On the Behavior of Some Oxides with Caustic Potash in the 
Presence of Oxide of Chromium" by Northcote and Church, Vol. 6, p. 54 
(1853). 



IRON TANNAGE 59 

Section IX.. Chrome-Iron Joint Tannage. 

This form of joint tannage from the use of sodium dichromate 
as the oxidizing agent for iron proved to be very successful. It 
did not give a pure iron tannage, but a joint tannage of the chrome 
as well as iron. The relative amount of tannage due to each in 
the resulting leather is dependent upon the relative quantity of 
each present in the liquor. 

From the invention of the Augustus Schultz's two-bath chrome 
process, it has been established that a dichromate, or chromium in 
the hexavalent state, has little or no tanning property until after 
it is reduced to the chromic (trivalent) state, Cr+ ++. It was 
seen* that ferrous iron had no tanning property until after it was 
oxidized to the ferric state. Considering the properties of the 
two salts it is evident that a combination of the two is a natural 
outcome, using one as the oxidizing agent and the other as the 
reducing agent, both being benefited by the reaction mutually en- 
gaged in so that a joint tannage results. From the chrome tan- 
nage point of view, the use of the ferrous salt as a reducing agent 
possesses some advantages over the other reducing agents such 
as sodium bisulphite, sodium thiosulphate, sulphurous acid, glu- 
cose, glycerine, etc. For, unlike these latter which generally leave 
inert substances in the bath after the reduction reaction and which 
contribute nothing beyond the reduction of the chromate,f the 
ferric salt formed from the reduction reaction of the ferrous salt 
can be utilized as a tanning agent in the same bath. From the 
iron tannage point of view, the choice of the dichromate as an 
oxidizing agent is prompted by many considerations. First, as 
an oxidizing agent its oxidation potential is high and the oxida- 
tion reaction rapid, proceeding to completion in the cold. Second, 
for its oxidation action it requires only a very low acidity^ in the 
solution so that the basicity of the resulting iron liquor is com- 
pletely under control. Third, the waste product from the oxida- 
tion reaction, namely, the chrome salt, is a valuable tanning agent 
which can contribute fully to its share in the resulting tannage. 
Fourth, the green color of the chrome tannage has the effect of 

^Section 8, page 54, footnote. 

t In case sodium thiosulphate is used as the reducing agent, the col- 
loidal sulphur may contribute some tannage, and it gives a lighter color 
to the leather. 

t Contrast the case where a chlorate is used as the oxidizing agent. 



6o i,e;athe;r chemists association 

neutralizing the red-yellow color of the iron, yielding a product 
of a less pronounced color. One possible drawback in the use of 
iron as the reducing agent for the chromate might be that the 
quantity of the ferrous salt used is comparatively large (5j^ parts 
of FeSO^^HgO to i part of Na2Cr207.2H20 by weight) espec- 
ially when the commercial copperas has been partially air-oxi- 
dized, and that the color of the product is somewhat darker 
(brownish) than when other reducing agents are used with the 
chromate (light green). But for a certain class of goods this 
is not objectionable, and advantage can well be taken of the lower 
cost of production. It is important that the basicity of the bath 
be carefully adjusted,* otherwise the bath may be either too acid 
for the chrome or too alkaline for the iron, so that joint tannage 
can not be brought about. In general, an amount of 66° Be. sul- 
phuric acid equal to from 30-35 per cent, of the copperas em- 
ployed with a sufficient amount of the sodium dichromate for 
complete oxidation is found to work well. 

The following procedure may illustrate the mode of tannage 
in actual tannery practice. The percentages given are all calcu- 
lated on the basis of the weight of the drained, pickled pelt (sheep- 
skins, calfskins, etc.). For convenience, the weight of the skins 
is taken as basis to figure the quantities used. By "gal. %" (gal- 
lons per cent.) is meant gallons of the liquid in question per 100 
pounds of the skin. For goatskins a somewhat larger quantity 
should be taken, say 10-20 per cent, greater. The examples given 
apply to drum tanning. 

Pel cent. 

(I) Copperas (FeS04.7H20) .'.. 11 

Salt (NaCl) 5 

Sulphuric acid (66° Be. H2SO4) i^ 

Water for solution (total) 12 gal. 

Drum pelt in the solution for ^ hour, then introduce a solution of 

Per cent. 

Sodium dichromate (Na2Cr207.2H20) . . 2% 

Water to dissolve 2 gal. 

* Historically Hylten Cavalin came close to the process, but because 
of lack of proper adjustment for the acidity he failed to obtain a success- 
ful tannage (Section 2, page 72>)- 



IRON TANNAGE , 6l 

Drum for about ij^ hours. (See if all iron is oxidized.) Add 
very slowly in portions, preferably through the trunnion, a solu- 
tion of 

Per cent. 

Soda ash (Na^CO^) 45^ 

Water to dissolve 3 gal. 

After all the alkali is in, drum for 10 minutes longer. (See if the 
pelt is neutralized.) Rinse. This gives a tannage more of the 
nature of the iron than the chrome. The following modification 
can also be employed, if desired. 

Per cent. 

(II) Sodium dichromate (NaaCraOr.aHjO) 2J4 

Salt (NaCl) 5 

Water for solution 12 gal. 

Drum the pelt in the solution for 3/^ hour. Add to the drum a 
solution of 

Per cent. 

Copperas (FeSO^.yHjO) 12 

Sulphuric acid (66° Be. H2SO4) 2^ 

Water for solution 4 gal. 

Drum for I J^ hours. (See that all chrome is reduced.) Runout 
excess spent liquor. Then introduce a suspension of 

Per cent. 

Bleaching powder V/z 

Water 3 gal. 

Drum for 15 minutes. Introduce very slowly as before a solution 
of 

Per cent. 

Soda ash 3^ 

Water to dissolve 3 gal. 

After all alkali is in, drum for 10 minutes longer. (See if the 
pelt is neutral.) Rinse. This gives a tannage more of the nature 
of chrome than iron. 



62 IvEATHKR CHEMISTS ASSOCIATION 

(III) For one-bath tannage. 

(a) When the liquor is to be prepared, take for each loo pounds 

of the pelt 

Per cent. 

Copperas (FeS04.7H20) li 

Sulphuric acid (66° Be. H2SO4) 3 

Salt (NaCl) 5 

Sodium dichromate (NazCraOT.sHjO) 2^ 

Water (total) for solution 15 gal. 

(Add the dichromate very slowly- when stirring. Use the liquor 
without unnecessary delay.) 

(b) When a concentrated one-bath is already made according to 

the method of preparation given in Section 3, take 

Per cent. 

Chrome-iron liquor (concentrated) 3 gal. 

Water to dilute 12 gal. 

Soda ash (NajCOa) to neutralize J/^ 

In either case, drum the pelt in the liquor for i to i^ hours, or 
until the pelt is struck through. 

Introduce very slowly as before 

Per cent. 

Soda ash (NaiCOa) 5 

Water to dissolve 4 gal. 

After all the alkali is in, drum for 10 minutes longer. (See if 
the pelt is neutral.) Rinse. 

The stock tanned by any of the above processes should be soft 
and full. It has a color varying from a dull yellow, grayish 
brown, to olive drab, depending upon the proportion of the chrome 
to the iron present. To secure the predominating effect of the 
chrome tannage, some chromic salt may be added to the liquor. 
The leather obtained does not stand boiling unless the chrome 
content is increased by the addition of a chromic salt to the liquor. 

The leather can be dyed black with logwood with or without a 
"striker." It can be dyed with coal tar dyes, such as the acid, 
direct, and alizarine dyes. When it is to be dyed with a basic 
dye a mordant is required, in which case, the tanned stock is best 
first dried to "crust" and then wetted back for mordanting with 
fustic or other vegetable matter after which the basic dye is 
applied in the usual manner. For fat-liquoring a somewhat larger 



IRON tannage; 63 

quantity of the fat-liquor (5-8 per cent, of "sulphonated" cod 
liver oil, degras, "sulphonated" Neat's foot oil, or a commercially 
prepared mixture) can be used. The proper temperature for dye- 
ing is between i3o°-i40° F. and that for fat-liquoring iio°- 
120° F. 

The leather can be re-tanned in oil to advantage when cod liver 
oil or other fish oil may be used. It can be re-tanned and colored 
black in ordinary vegetable tannins. Some basic black can be 
used for topping. When a less astringent tannin is used, a light 
gray color is obtained. In such cases, drying to "crust" prior to 
the treatment is advisable. 

A sample of sheepskin leather tanned according to (I) above 
gives the following analysis : 

Per cent. 

Moisture 7.43 

Ash 15.69 

Fat 21.48 

Fe203 10.51 

CnOa 1.84 

*P205 2.50 

SO3 (total) 2.12 

Hide substance (N X 5-62) 43.85 

* From some disodium phosphate introduced together with the alkali 
for neutralization. 



64 i,e:athe;r che;mists association 

Section X. Pure; Iron Tannage;. 

As it is desired to determine the actual tanning value of a ferric 
salt, a considerable portion of the time has been devoted to the 
study of the pure iron tannage, that is to say, to the tannage 
where no other metals except iron that can yield a tannage are 
present. It has been often reported that iron tannage produces a 
brittle leather, a leather that draws together on drying, a leather 
that deteriorates on keeping, and so on. One of the arguments 
advanced is that iron in the leather acts as an oxygen carrier, 
taking in oxygen from the air and imparting it to the fiber, so 
that the fiber is gradually oxidized and corroded in the course of 
time.'^^ It has so far not been found possible to confirm this re- 
port, but, on the other hand, there is sufficient evidence to show 
that any defect of this kind is due to an improper tannage rather 
than to the inherent nature of the tannage. For, when a leather 
is properly tanned, it is not at all brittle, does not draw together 
hard on drying, nor behave in any way different from other min- 
eral tannage. Samples of sheepskin leather tanned with iron salts 
that have now been kept for more than ten months show no sign 
of deterioration of the sort reported. It is probable that these 
defects were brought about by the use of a too alkaline iron liquor 
in which ferric oxide had been caused to deposit on the surface, 
making the interior of the pelt impenetrable to the tanning agent. 
Too rapid a neutralization would also cause the same defect, as 
the ferric oxide which is caused to separate from the solution 
would coat the surface of the pelt. This gives rise to a hard 
outer layer (grain) and a wide soft zone underneath in the cross 
section of the pelt. 

As it is the ferric iron that possesses the tanning property, it 
follows that all iron should be kept in the ferric state. It is not 
so much, however, to avoid a small loss of iron going to the fer- 
rous state as to prevent the ferrous iron finding its way to the 
■ pelt causing irregularities in appearance and texture in the leather. 
Hence it is necessary to use a sufficient quantity of a proper oxi- 
dizing agent to bring about complete oxidation, and not only that, 
to use a small excess of the oxidizing agent (10-15 P^^' cent.) 
to take care of any subsequent reduction. To insure this, a fur- 
ther guaranty is found in the introduction of a small ciuantity of 
^^ "Die moderne Leder-Fabrikation" by Hermann Zeidler, p. 109 (1914). 



IRON TANNAGE 65 

a suitable oxidizing agent (Na2Cr207.2H20, CaClO.Cl, etc.) to- 
ward the end of the tanning process, prior to the neutralization — 
the so-called after oxidation. This is a proper action at this stage, 
inasmuch as the oxidation reaction involves a decrease of hydro- 
gen ion concentration in the solution, thus helping to fix the iron 
in the pelt . 

The best basicity for the tan liquor is found to lie in a range 
varying between the ratio of 5 equivalents of the mineral acid 
radical (or radicals) present to every 6 equivalents of the ferric 
iron, and that of 3 equivalents of the mineral acid radical (or 
radicals) present to every 4 equivalents of the ferric iron. When 
much iron salt in the liquor is in the form of a sulphate, it is 
safer not to go too near the higher limit of basicity. For if such 
is the case a light yellow hydrated ferric oxide (not a red gela- 
tinous ferric hydroxide) would then be thrown out on short 
standing. It is evident that the same danger of rapid precipita- 
tion exists during neutralization. Hence it is necessary to effect 
the neutralization very gradually. The total amount of an alkali 
needed for neutralization is only 70-80 per cent, of the theoretical. 

For the oxidation of iron and the preparation of the tan liquor 
from^a ferrous salt, chlorine is found to work very satisfactorily, 
as it can effect the oxidation in the cold in -the absence of any 
acid and push the reaction to completion under a small pressure.* 
Other sviitable methods of oxidation are those using nitric acid 
and sulphuric acid, and sodium nitrate and sulphuric acid. The 
latter is more economical because of the cheapness of the sodium 
nitrate (Chile saltpetre) employed. All methods involving the 
use of nitric acid in one form or another, however, require a 
boiling temperature, and hence a special container to resist the 
corrosive action of the hot nitric acid. 

For tanning, a quantity of ferrous sulphate crystals, FeS04.- 
7H2O, between 12 and 15 per cent, of the weight of the drained 
pelt is generally sufficient, the higher figure being for heavy hides 
and for the goat skin. A rough guide to secure the correct bas- 
icity for tanning is to add 10-14 per cent, of soda ash, Na2C03, 
of the weight of the ferrous sulphate crystals taken. This pre- 
sumes that the ferric liquor to start with is neutral in composition. 

* For detailed directions concerning the preparation of the tan liquor, 
see Section 3. 



66 i,e;athe;r chemists association 

The following method for sheepskins in drum tanning can be 
used for illustration. Unless otherwise stated, all percentages are 
on the basis of the weight of the drained pelt. When the stock 
to be tanned is much below lOO pounds some judgment should be 
exercised in regard to the modification of these percentages. 

Per cent. 

Iron liquor containing an amount of FeaOs equal to 3^ 

(or as FeS04.7H20 12) 

Salt, NaCl 4 

Soda ash, Na^COg l^ - 

Total volume for tanning 25 gal. 

Drum for i to i^ hours. Introduce into the drum a suspension 
containing 

Per cent. 

' Bleaching powder, CaCIO. CI i^ 

Water I gal. 

Drum for 15 minutes longer. Neutralize the pelt very gradually 

(in small portions) with a solution of 

Per cent. 

Soda ash, Na2C03 4 

Water to dissolve 3 gal. 

After the alkali is all fed in, drum for 10 minutes longer. Rinse. 
Hang the tanned pelt to dry. Sammy back from "crust" and wet 
thoroughly for subsequent operations. 
For a coloring black, use ^ 

Per cent. 

Hematine crystals i ^ 

Water to dissolve equal to twice the weight of the wet 
stock 

Make the solution alkaline with ammonia, and heat to 130° F. 
Drum for 30 minutes and then add to the drum a solution warmed 
to 130° F. containing 

Per cent. 

Basic leather black i 

Water to dissolve 5 gal. 

Drum for 20 minutes, or until the leather is colored through. 
Run off the spent dye liquor. Fat-liquor with an emulsion at 130° 
F. containing 



IRON TANNAGE 67 

Per cent. 

"Sulphonated" cod liver oil 6 

Water 80 

Drum for 45 minutes, or until all fat-liquor is taken up. Hang 
the fat-liquored stock to dry without setting out. Any commer- 
cial fat-liquor mixture can be used. 

For such a black leather, however, a re-tan in ordinary vege- 
table tannins is more economical and advantageous, since the veg- 
etable tannin not only gives a black color but also a tannage to 
the leather. Consequently the leather obtained is fuller. For such 
purpose, use a tannin liquor, warmed to 110° F., containing, say. 

Per cent. 

Quebracho liquid extract 15 

Water 120 

or a tannin liquor having a barkometer reading of from 15° to 
20° Bk. Drum for i^ to 2 hours. This generally gives a gray- 
ish black color. To obtain a deeper black color, top the leather 
with I per cent, basic leather black in the usual manner. This 
method of blacking dispenses with the logwood color and even 
with the "iron striker." For a light (silver) gray color, pyro- 
gallol tannins, such as pure oakwood tannin, sumac, etc., or a less 
astringent catechol tannin, such as mimosa, gambler, etc., can be 
used. This vegetable re-tan, however, can best be carried out in 
a paddle. 

In the case of heavy leather, an oil re-tan can be applied to ad- 
vantage, using, say, 

Per cent. 

Degras 8 

"Sulphonated" cod oil 4 

Water 12 gal. 

Drum for i^ hours with the mixture warmed to 130° F. (At 
present shark liver oil is available and can be utilized for this oil 
re-tan.) Or, the leather may be stuffed with a mixture of stearin, 
tallow, and "sulphonated" cod oil, using, for example, 

Per cent. 

Stearin 4 

Tallow 10 

"Sulphonated" cod oil 8 



68 i,^athe;r che;mists association 

Heat the fat mixture to 150° F. in the drum, and drum for about 
Yz hour. 

In general, an iron tanned leather is tough, heavy, but some- 
what harsh. Hence it is generally advantageous to give the 
leather a good far-liquoring, or oil stuffing, or oil re-tan. It does 
not resist a boiling temperature but begins to contract at about 
170° F. or lower. 

For dyeing the iron tanned leather with basic dyes, the remarks 
made in Sections 8 and 9 apply here with a greater force. 

A sheepskin leather tanned with 8 per cent. Fe2( 804)3, 4 per 
cent. NaCl, and 1.6 per cent. NagCOg, subsequently neutralized 
with 4^ per cent. CaCOg, and finally lightly fat-liquored with a 
mixture of neat's foot oil and a mineral oil, gives the following 
analysis : 

Per cent. 

Moisture 14.10 

Fat 5.37 

Ash 20.01 

Fe203 4.08 

SO3 (total) 3.26 

Hide substance 51.22 (N X 5-62) 

This sample of sheepskin leather is tough and full, but feels some- 
what harsh. It has a beautiful yellow-red color. From the analy- 
sis of the iron content, it seems that an amount of iron as low 
as 4 per cent. FcgOg of the weight of the air-dried sample is 
sufficient to give a satisfactory tannage. 

Calcium carbonate (or the "precipitated lime") or magnesium 
carbonate is found to be very suitable for neutralization in place 
of, or together with, soda ash. It is cheap and can be used in 
excess to prevent the presence of any free mineral acid (H2SO4) 
in the leather. The calcium or magnesium sulphate formed in the 
leather during neutralization, furthermore, serves to give weight 
to the leather. 

It might also be mentioned that in a tannery v/here chrome or 
vegetable tannage is employed, the presence of an iron salt is in- 
compatible with good appearance of leather and all possible care 
is to be taken to keep away any iron from all liquors. In iron 
tannage this difficulty disappears. 



IRON TANNAGE 69 

Section XI. — Iron Phosphate; Tannage. 

As the iron-tanned leather has a pronounced red-yellow color 
and as iron is capable of forming many colored bodies both with 
the organic and inorganic substances, attention is naturally drawn 
to the utilization of this chemical activity of iron for coloring 
the leather by a treatment with a substance which combines with 
iron to give the color. At the same time, it should be equally 
possible to find a substance that will give with iron a color lighter 
than that of the iron leather itself. In general, these combina- 
tions are in the form of a precipitate. Thus, Knapp treated his 
iron leather with a fatty acid forming a yellow precipitate of 
"iron soap" in the leather.^- He was not, however, so much con- 
cerned with the yellowish brown color of the "iron soap" as with 
the fixation of iron in the leather so that it could not be washed 
out. The use of potassium ferrocyanide solution for producing 
a blue color has been mentioned.^^ But it is found that the color 
is not fast and is gradually washed out, especially when the leather 
after such a treatment is not immediately fat-liquored; and, if 
the pelt is treated with the ferrocyanide solution before the iron 
has been fixed in the leather, a very poor tannage is obtained. 
Logwood (for dull black color) and fustic (for green-black 
color) coloring matters have been long known in their use with 
iron (as a "striker") ; but while they give a fast color, they con- 
tribute no material tannage. And it is said that much excess of 
iron should be avoided as it would render the leather brittle and 
liable to crack. -''* A soluble sulphide or polysulphide has been 
advocated for making black leather in connection with the iron 
tannage by O. Rohm,^^ but there are grave doubts as to its prac- 
ticability, because these alkaline or alkaline earth sulphides gen- 
erally have a strong reducing action, and the black ferrous sul- 
phide formed is merely a filler and not a tanning agent for the 
pelt. 

It is found feasible, however, to make the black or grayish 
leather by re-tanning the iron leather in ordinary vegetable 
tannins. This not merely gives the color, but also involves a 

''British Patent No. 2,716 (1861). 

'"'A Text Book of Tanning," by H. R. Procter, p. 222 (1885). 
'^"Leather Dressing," by M. C. Lamb, p. 149 (1907). 
''British Patent No. 103,827 (1917). 



•JO I^^ATHSR CHEMISTS ASSOCIATION 

further tanning action. The leather thus obtained partakes of 
the characteristics of both the mineral and the vegetable tannages. 

A scheme of making a light colored, or substantially white 
leather with the use of a phosphate has been satisfactorily worked 
out. The function of the phosphate seems to be more than pro- 
d.ucing a yellowish white compound of ferric phosphate in the 
leather. Borax having a property of forming a light red com- 
pound, ferric borate, could be used, but the quantity needed is 
usually large. It is, however, suitable for neutralization because 
of its mild alkaline nature. It may be added that other white or 
yellowish white compounds of iron are the ferric arsenate, and 
the iodate. Certain features about these substances, such as the 
poisonous character, the cost, etc., make their use for leather 
making clearly impracticable. 

The idea of using a phosphate in connection with the iron tan- 
nage was suggested by the fact that from the colloid chemistry 
point of view the phosphate ion, being a trivalent negative ion, 
should act favorably towards the fixation of iron in the pelt. 
The mode of procedure is illustrated by the processes given below : 

I. The pelt to be tanned is drummed in the ferric iron liquor 
of the same character and basicity as for pure iron tannage* and 
treated in the same manner up to the neutralization of the pelt. 
For neutralization, use 

Per cent. 

Sodium pyrophosphate, Na4P207.,ioH20 4 

Soda ash, NazCOj 2^4 

Water for solution 20 

or 

Trisodium phosphate, NasPO*. 12H2O 6 

Soda ash i^ 

Water, same as above 
Introduce the solution slowly in the usual manner. It is found 
better to introduce the carbonate together with the phosphate than 
to add the phosphate alone first. Pyrophosphate is preferred 
because of its high phosphate content for a given weight. Borax 
may be used together with the phosphate and the carbonate; in 
which case the amount of the latter used should be correspond- 
ingly decreased. This tannage gives a leather of a light color. 
Subsequent treatments given in Sections IX and X can be fol- 
lowed. 

* See Section 10 on Pure Iron Tannage. 



IRON TANNAGE 71 

II. The phosphate may be directly added to the ferric tan 
liquor giving a fine milky suspension. In this case, the addition 
of the alkali to bring about the proper basicity for tanning should 
be omitted. For sheepskins in drum tanning, use 

Per cent. 

Ferric salt (calculated as Fe2(S04)3) 9 

Sodium pyrophosphate, Na^PzO?. 10H2O 4 

Total volume 15 gal. 

Dissolve the pyrophosphate in a small quantity of water and add 
it to the ferric salt solution slowly with constant stirring. Hav- 
ing stirred thoroughly, introduce it immediately into the drum 
and tan the pelt for 3 to 5 hours, or until the pelt is thoroughly 
penetrated. Neutralize the pelt slowly with 

Per cent. 

Soda ash, NajCOa 3^ 

Water to dissolve 3 gal. 

After all the alkali is fed in, rinse and hang to dry. This gives 
an especially white leather. The. process is suitable for light 
stock, such as glove leather and the like. The penetration is 
somewhat slow, hence a longer drumming is needed. When dried, 
the leather is full and soft. It becomes velvety after staking and 
perching. 

The leather can be finished by any of the usual treatments. In 
dyeing, with basic coal tar dyes, the ordinary precautions in con- 
nection with the use of vegetable mordants should be observed. 
As the leather does not resist a high temperature, it is important 
not to use a temperature above 140° F. in dyeing or fat-liquoring. 

A sample of sheepskin leather treated in accordance with (II) 
above, but without neutralization or fat-liquoring, gives a soft 
and almost white leather. Its chemical analysis gives 

Per cent. 

Moisture 11.48 

Fat 11.75 

Ash 12.23 

Fe^Os 3-97 

P2O5 2.32 

SO3 (total) 2.15 

Hide substance (N X 562) 54.90 

Here again it shows that about 4 per cent, of iron as Fe^Og on 
the basis of the air-dried sample is sufficient to give a tannage for 
light skins. 



J2 tE^ATHER CHEMISTS ASSOCIATION 

Se;ction XII. — CoNcivUsioNs. 

The character of the iron tannage seems to lie between that 
of the alum and that of the chrome tannage. Iron seems to 
yield a more permanent tannage (towards water) than alum, 
but like the alum tannage, iron tanned leather does not resist the 
boiling temperature of water. If we take the critical temperature 
as that at which the sample under water begins to shrink or to 
draw together under the influence of heat, that point generally 
lies between i6o°-i75° F. In the case of a re-tanned leather (in 
fish oils or vegetable tannins) a somewhat higher test may be 
obtained; but in no case can an iron-tanned leather stand boiling, 
unless considerable portion of the tannage is due to chrome as in 
the case of the chrome-iron joint tannage. 

It has been often reported that iron-tanned leather produces a 
brittle grain, and rots on storing. To do justice to the iron tan- 
nage it must be declared that an iron-tanned leather, properly 
tanned, is not brittle on the grain and does not deteriorate on 
storage. Samples of the leather that have now been kept for 
more than ten months show no sign of deterioration. Sometimes 
the product obtained is somewhat stiff and "flat," but this should 
not be ascribed to the inherent properties of the tannage. The 
strength, the fullness, the elasticity are, in our opinion, a matter 
of proper tannage and not dependent upon the nature of the 
tannage. 

As a considerable amount of salt (4-5 per cent, of the weight 
of the pelt) is needed in the liquor and much of it is formed from 
neutralization, it is important to rinse the tanned stock after neu- 
tralization to wash off most of the neutral salts present (NaCl, 
NagSO^, etc.) ; otherwise their presence in the leather may cause 
dampness or even salt stains or spues. Iron tannage is much 
affected by the presence of grease or any imperfections in the 
skins, and when such is the case, unevenness of color and other 
irregularities are liable to show up on drying. Hence the neces- 
sity of uniform softening of the pelt and of degreasing. 

Iron-tanned leather generally runs high in ash. The leather 
has oftentimes a harsh feel, due probably to the presence of a 
large amount of iron oxide (FcgOg) in the leather. Because of 
the harsh feel it is generally advisable to give the leather a some- 



IRON TANNAGE 73 

what heavy fat-liquoring or an oil treatment. The use of flour, 
egg yolk, etc., may be practiced, if desired. At the present stage 
of our knowledge it seems that to produce a satisfactory tannage 
at least for a light leather an a'mount of iron calculated as FcgOg 
not less than 4 per cent, of the weight of the air-dried sample 
should be present. 

The iron-tanned leather compares favorably with other mineral 
tanned leather. The red-yellow or brown-red color of the tan- 
nage, however, is for some purposes an undesirable feature. The 
chemical activity of iron in the leather forming dark colored com- 
pounds in the leather is another drawback. But even with all 
these limitations, there is much to be said in its favor. There 
are certain classes of goods in which these features are of no 
consequence and the saving in the cost of production is very 
material. True, there are difficulties in connection with the tan- 
ning operation and subsequent treatment of the leather — difficul- 
ties which in other tannages either do not exist, or are less serious. 
But the process, like any other new process, necessitates a new 
set of conditions. To summarize, the following main factors may 
be mentioned : 

I. Completeness in the oxidation of iron and maintenance in 
its ferric state by using an excess of a proper oxidizing agent, and 
by means of an after oxidation. 

II. Adjustment of proper basicity by the addition of a proper 
amount of an alkali, a basicity between the ratio of one OH" 
equivalent to every 5 equivalents of the mineral acid radical pres- 
ent, and that of one OH" equivalent to every 3 equivalents of 
the mineral acid radical present, being the proper range for 
tanning. 

III. Gradual neutralization to be eflfected so that iron may be 
uniformly fixed in the pelt throughout its thickness. 

IV. Drying to the "crust" state before subsequent treatment to 
minimize the chemical reactions between the iron in the stock 
and the substances employed that would react with iron to give 
an undesirable color. 

It should not be omitted to mention that the subject of iron 
tannage presents a broad unexplored field and that this study is 
far from being exhaustive. Other phases could have been taken 
up and it is hoped that this work will serve as an indication for 
much that remains to be done. 



74 leather chemists association 

Bibliography. 
The More Important Works in Leather Industry. 

/. Books on Leather Manufacture. 

1. Leather Dressing, by M. C. Lamb. 

G. Sadler & Co., London. 

2. Handbuch der Chromgerbung, by Josef Jettmar. 

Schulze & Co., Leipzig. 

3. Die Rotlederfabrikation, by Joseph Borgmann. 

M. Krayn, BerHn. 

( I. Teil, Die Unterlederfabrikation.) 
(II. Teil, Die Oberlederfabrikation.) 

4. The Principles of Leather Manufacture, by H. R. Procter. 

E. & F. N. Spon, Ltd., London. 

5. Die Chromgerbung, by Joseph Bergmann. 

M. Krayn, Berlin. 

6. Practical Tanning, by Louis A. Flemming. 

Henry Carey Baird & Son, Philadelphia. 

7. Modern American Tanning (in two volumes). 

Edited by Jacobsen Publishing Co., Chicago. 

8. Praxis und Theorie der Leder-Erzeugung, by Joseph Jettmar. 

Julius Springer, Berlin, 
p. The Manufacture of Leather, by H. G. Bennett. 
Constable & Co., London. 

10. Die moderne Lederfabrikation, by H. Zeidler. 

Bernh. Tr. Voigt, Leipzig. 

11. La Tannerie, by Louis Meunier and Clement Vaney. 

Gauthier-Villars, Imprimeur. 

Libraire de I'Ecole Pol3i:echnique, Paris. 

12. Die Feinlederfabrikation, by Joseph Bergmann. 

M. Krayn, Berlin. 

13. A Text Book of Tanning, by H. R. Procter. 

E. & F. N. Spon, Ltd., London. 

14. Leather, by K. J. Adcock. 

Sir Isaac Pitman & Sons, Ltd., London. 

15. Leather Worker's Manual, by H. C. Standage. 

Scott, Greenwood & Son, London. 

16. The Making of Leather, by H. R. Procter. 

Cambridge University Press, England. 

17. Leder-Fabrikation, by H. Kronlin. 

Max. Janecke, Hannover. 

18. Leather Manufacture, by Alex. Watt. 

Crosby, Lockwood & Son, London. 

19. The Manufacture of Leather, by Chas. T. Davis. 

Henry Carey Baird & Co., Philadelphia. 

20. Laboratory Guide of Industrial Chemistry, by Allen Rogers. 

D. Van Nostrand Co., New York. 



IRON TANNAGE 75 

21. Practical Treatise on the Leather Industry, by A. M. Villon; 

translated by F. T. Addyman. 
Scott, Greenwood & Son, London. 

22. The Art of Tanning, by Campbell Morfit. 

Henry Carey Baird & Co., Philadelphia. 

23. La Chimie du Cuir, by Lion Eglene. 

H. Dunod et E. Pinat, Paris. 

II. Books on Leather Chemistry. 

1. Leather Industries Laboratory Book, by H. R. Procter. 

E. & F. N. Spon, Ltd., London. 

2. Leather Trades' Chemistry, by S. R. Trotman. 

Chas. Griffin & Co., Ltd., London. 

3. Handbuch fiir Gerberei-Chemische Laboratorien, by G. Grasser. 

Schulze & Co., Leipzig. 

4. Leather Chemists' Pocket-Book, by H. R. Procter. 

E. & F. N. Spon, Ltd., London. 

5. Practical Leather Chemistry, by Arthur Harvey. 

Crosby, Lockwood & Son, London. 

6. Tanners' and Chemists' Hand-Book, by L. E. Levi and E. V. 

Manuel, Milwaukee. 

///. Scientific and Commercial Periodicals of Leather Industry. 

1. Journal American Leather Chemists' Association. 

Easton, Pa., U. S. A. 

2. The Leather Manufacturer. 

Boston, Mass., U. S. A. 

3. Le Cuir, Edition Technique. 

Paris, France. 

4. Journal of the Society of Leather Trades' Chemists. 

London, England. 

5. Collegium, 

Haltigen, Germany. 

6. Ledertechnische Rundschau. 

Berlin, Germany. 

7. Der Gerber. 

Prague, Czecho-Slovakia. 

8. Color Trade Journal. 

New York, N. Y., U. S. A. 

9. Shoe and Leather Reporter. 

Boston, Mass., U. S. A. 

10. Hide and Leather. 

Chicago, 111., U. S. A. 

11. The Leather World. 

London, England. 

12. Shoe and Leather Journal. 

Toronto, Canada. 



^6 i^EATHER che;mists association 

IV. Miscellaneous. 

1. The Chemical Constitution of the Proteins (in two parts), by 

R. H. A. Plimmer. 
Longmans, Green & Co., London. 

2. The Physical Chemistry of the Proteins, by T. B. Robertson. 

Longmans, Green & Co., London. 

3. Die Gerbstoffe, by J. Dekker. 

Gebriider Borntaeger, Berlin. 

4. The Puering, Bating and Drenching, by J. T. Wood. 

E. & F. N. Spon, Ltd., London. 

5. Hides and Skins. 

Shoe and Leather Weekly, Chicago, 111. 



IRON TANNAGE ^^ 

Appendix. — A Tentative Procedure for the Ordinary 
Chemicai, Analysis of Iron-Tanned Leather. 

With more new chemicals introduced in the manufacture of 
leather, the chemical analysis of the leather naturally becomes 
more complicated. The following is a proposed system of the 
chemical analysis for iron-tanned leather ordinarily sufficient for 
commercial work. With the exception of the determination of 
free mineral acid, all procedures here given have been tested 
and found to give satisfactory results. While there are but few 
features in these methods, the details of the procedure, and the 
quantities of the reagents to be taken, etc., are those actually 
found to work well. The determination of the free mineral acid 
is based on the Procter and Searle's method, and that of the hide 
substance is adapted from the Dyer's modification of the Kjeldahl 
method for nitrogen. In order to bring out certain points in 
the analysis more clearly, notes have been added to each pro- 
cedure, based upon the results of observations in the laboratory. 
The order and the grouping of the determinations as found to 
be convenient are shown as follows : 

(1) Moisture ) ^^ ^^^ ,g 

(2) Fat S 

(3) Ash -j 

(4) FcsOs !- in one sample 

(5) Cr.Oa J 

(6) Free mineral acid | 

(7) P2O5 1- in one sample 

(8) SO3 (total) J 

(9) Hide substance 

When only isolated determinations are desired, this order, of 
course, need not be followed. 

Sampling. — Leather to be analyzed should be reduced to small 
pieces of approximately uniform size. Heavy leather can be 
shaved with a planer and ground in a small mill. Light leather 
should be chipped or shredded to pieces of about >4 inch long by 
1/16 inch wide with the natural thickness of the skin. A com- 
posite sample should be made from different parts of the whole 
piece and the sample intimately mixed before a portion is taken 
for analysis. The prepared sample should be kept in a tightly 
stoppered bottle. 



78 IvEATHEiR CHEMISTS ASSOCIATION 

1. Moisture. — Weigh 8 grams of the air-dried sample into a 
tared glass dish and dry for 8 hours in an electric oven regulated 
at 99°- 101° C. The loss in weight represents moisture. 

^■D <- • 4- v/ loss in wt. 

(Per cent, moisture = 100 X 7-.) 

wt. sample 

Note i. — ^The leather should not be exposed to a higher tempera- 
ture or heated for an unnecessary length of time because 
any drying oil (cod liver oil, shark liver oil, etc.) used for 
fat-liquoring, oiling, stuffing, or re-tanning would be oxi- 
dized to a greater extent. This not only gives low result 
for moisture, but also for fat determination, as petroleum 
ether will not dissolve the oxidized fat. 

Note 2. — The dried sample should be weighed rapidly as it quickly 
absorbs moisture from the air. 

2. Fat. — Transfer the sample from the moisture determination 
to a Soxhlet extractor using petroleum ether (redistilled if neces- 
sary using distillate below 60° C). Fill the dry, clean Soxhlet 
flask with 160-180 cubic centimeters petroleum ether (to about 
three-fourths full). Heat the flask in an electric heater (or over 
a water bath) for 8 hours after which distill the main portion of 
the ether from the flask into the thimble chamber, collecting this 
portion. Transfer the ether solution of the fat to a tared evapo- 
rating dish, evaporate off most of the ether over a steam bath, and 
dry the fat at 99°-ioi° C. in an electric oven for 2 hours. The 
content of the dish is fat. 

.^ ^ . . wt. of fat . 

(Per cent, fat = 100 X r •) 

wt. sample 

Note i. — To prepare the thimble for extraction, wash the thimble 
(S. & S.) in a small portion of the ether. Line the bottom 
of this thimble with a tuft of absorbent cotton that has 
also been washed in the ether. Place the sample in the 
thimble and cover it with another tuft of the washed 
cotton. This size of the sample, together v/ith the cotton 
lining and covering will be just comfortably contained in 
the thimble. The thimble prepared in this way will prevent 
any fine particles of leather from being sucked out through 
the bottom during syphoning, or from floating off the top 
when the thimble is completely covered by the ether. A 
piece of heavy glass tubing can be placed at the bottom 
of the chamber underneath the thimble to allow some 
clearance so that the ether can be completely drained from 
the thimble during syphoning. A small tuft of cotton may 



IRON tannage; 79 

be loosely placed at the opening of the condenser above. 

Note 2. — The same precaution in drying given in Note i under 
moisture determination applies here. When mineral oil is 
present in the fat extraction it is sometimes difficult to 
get a constant weight due probably to the mineral oil being 
constantly decomposed and volatilized oflf. The dried fat 
should also be weighed rapidly. 

Note 3. — There is only a trace of iron salt that is extracted from 
the leather by the ether. 

3. Ash. — Weigh 2 grams of the air-dried sample in a tared 
plaitinum dish, platinum crucible, or porcelain crucible and heat 
first very gently and then to below dull-red heat. Stir the con- 
tents occasionally with a platinum wire and heat gently until it 
is thoroughly ashed. The residue is weighed as ash. 

, ^ , ^ wt. of ash . 

(Percent, ash = 100 X r ■) 

wt. sample 

Note i. — Chlorides of metals are likely to be partially volatilized 
and lost at a higher temperature. Sulphates of heavy 
metals are decomposed with the evolution of SOj fumes. 
If the sample is heated too strongly, especially at the 
beginning, the leather cakes together so that the inner part 
is difficult to bum off. Sometimes the content fuses when 
heat is applied too strongly, so that it is hardly possible 
to transfer the ash to a crucible for alkaline fusion. In 
this case it is better to use a platinum crucible for the ash 
determination. But when a phosphate is present, great 
care should be taken not to cause the reduction of the 
phosphorus with the result of ruining the platinum crucible. 

Note 2. — For iron-tanned leather a 2-gram sample is sufficient, as 
the ash usually runs high. 

Note 3. — Owing to an inevitable loss of some chlorides and to an 
indefinite amount of sulphates decomposed, the significance 
of the ash determination cannot be of great value. 
Furthermore, unless the manner of heating and other con- 
ditions are the same, good checks in different hands are 
difficult. 

4. Iron. — The ash from the last determination is fused in a 
platinum crucible with a well pulverized and intimately stirred 
mixture containing i}^ grams anhydrous pure K2CO3, i^ grams 
anhydrous pure NaaCOg, and 13^ grams pure borax glass, until 
the liquid in the crucible appears homogeneous. Cool, meanwhile 
heating to boiling 150 cubic centimeters of distilled water in a 



8o i^EATHSR che;mists association 

350 cubic centimeter casserole. Place the crucible in the hot 
wat«r, cover the casserole with a watch glass, and boil very care- 
fully. Wash out the contents of the crucible, break up the mass, 
and allow to settle. Filter by decantation, and wash the precipi- 
tate with hot water, collecting the filtrate in a 400 cubic centi- 
meter beaker. Ignite the precipitate and weigh as FcjOg. 

^_ -^ ^ wt. Fe„0, . 

(Per cent. Fe^O, = 100 X — H^.) 

■^ ' wt. sample 

Note i. — Only a trace of iron is found to pass into the filtrate. If 
desired, the precipitate on the filter can be dissolved with 
20 cubic centimeters hot dilute HCl (i cone. HCl : 2 water 
by vol.) and the ferric hydroxide precipitated again with 
NH4OH with the addition of 2 grams NHiCI. Or, the 
iron in the HCl solution can be determined by the Zimmer- 
mann-Reinhardt volumetric method, taking care to oxidize 
ofif all organic m.atter with KMn04 before SnCU reduction. 

5. Chromium. — Cool the filtrate from the iron determination 
and make it up to 250 cubic centimeters. Pipette 100 cubic centi- 
meters of the filtrate into a 500 cubic centimeter beaker. Dilute 
to about 2CX) cubic centimeters. Acidify with concentrated HCl 
and add 5 cubic centimeters in excess. Add 15 cubic centimeters 
of 15 per cent. KI solution and titrate with N/io sodium thio- 
sulphate solution, adding i cubic centimeter of thin, clear starch 
solution after the color of the solution haS changed from red to 
light yellow. Titrate to the disappearance of the blue color. 

.^ ^ ^ . cc. N/io Na„S,0, X 0.002533 ^^ , 

(Per cent. Cr.Og = 100 X ' y ^^ X 2.5). 

^ . wt. sample 

Note I. — When the chromium present is small, the orange color of 
the dichromate cannot be distinguished. Hence the acidi- 
fication should be guided by a litmus paper. 

Note 2. — A thin, clear starch solution that can keep for several 
months is prepared as follows : Take i gram ordinary 
starch powder and rub it into a paste with 25 cubic centi- 
meters distilled water. Heat 200 cubic centimeters dis- 
tilled water to boiling and stir the thin paste into the hot 
water. Boil for a few minutes when a transparent solu- 
tion will be obtained. Filter the solution through absorbent 
cotton into a 250 cubic centimeter glass stoppered bottle. 
Add 5 cubic centimeters chloroform, stopper the bottle and 
shake. 

Note 3. — When chromium in the iron tan liquor is to be deter- 



IRON TANNAGE ol 

mined, pipette 25 cubic centimeters of the sample in a 
250 cubic centimeter graduated flask. Make up to the 
mark. Take 25 cubic centimeters and dilute to 35 cubic 
centimeters with distilled water. Oxidize the chromium 
with NazOa by adding small portions at a time with con- 
stant shaking; i>^ to 3 grams NajOa is sufficient for a 
sample containing 15 to 30 milligrams CrjOa. After all 
the Na^Oj has been added, heat the solution until the vol- 
ume remaining is about 10 cubic centimeters. Add 25 cubic 
centimeters distilled water and evaporate down to this 
same volume again. Dilute to about 150 cubic centimeters, 
bring to a boil, allow to settle and filter off the FcaO,, 
collecting the filtrate in a 350 cubic centimeter beaker. 
Wash the precipitate with hot distilled water, collecting 
it with the filtrate. Determine chromium in the filtrate as 
before.'" Ignite the precipitate and weigh as FezOa. 

(Remark: Excess of NaaOa used must be completely 

decomposed or a phenomena of the reappearance of the 

starch blue color shortly after it is discharged will occur, 

making the, determination worthless.)'' 

6. Free Mineral Acid (Based on Procter and Searle's Method). 

Weigh 2-gram sample in a platinum dish. Cover the sample 

with 25 cubic centimeters N/io NasCO, (accurately titrated 
against the HCl used below) . Allow the sample to wet thoroughly 
and evaporate to dryness on a water bath. Gently char the or- 
ganic matter, cover with about 50 cubic centimeters hot distilled 
water, stir, and break up the mass. Filter into a 250 cubic cen- 
timeter beaker. Return the residue with the filter to the dish, 
and ignite gently. Cool and take up the ash with 25 cubic centi- 
meters N/io HCl. Filter into the previous filtrate and wash 
thoroughly. Add 1-2 drops methyl orange, and if red color is 
seen, titrate with N/io alkali. 

(Per cent, free mineral acid — 

cc. N/io NaOH X 0.0049 ^^ tt qo ^ 

100 X ■ : i ^s npu^. ) 

wt. sample 

Note i.— This method has not been tested. A full discussion is 
found in "Leather Industries Laboratory Book" by H. R. 
Procter, pp. 367-73 (iQIQ)- From the experience with 
chrome leather analysis, we found that sometimes the color 
of the filtrate was so dark that it interfered with the 
methyl orange color in titration. 
'" See Section 4, p. 30, footnote. 

''" See "On the Volumetric Determination of Chromuim m Chrome 
Leather" by Te-Pang Hou, Jour. Am. Lea. Chem. Assoc, p. 367 (1920). 



82 i,e;athe;r chemists association 

7. Phosphate. — Ignite the residue from the last determination 
together with the filter paper in a platinum crucible. Fuse with 
I gram pure anhydrous K2CO3, i gram pure anhydrous NagCOj, 
and I gram pure borax glass until the liquid appears homogeneous. 
Dissolve out the content of the crucible as described under iron 
determination above, filter and wash, combining the filtrate with 
the solution from the alkali titration for Free Mineral Acid. 
Acidify the solution slightly with HCl. Add 2 grams NH4CI 
and 15 cubic centimeters magnesia mixture and heat to boiling. 
Cool in ice water and add ammonia very slowly at first until the 
solution smells of ammonia on stirring. Add one-fifth of the 
volume of the solution of concentrated ammonia and allow to 
stand at room temperature for about 30 minutes. Filter by de- 
cantation and wash with water to which 2-3 per cent, ammonia 
has been added. Ignite the precipitate in a tared porcelain 
crucible first gently and then strongly with a blast burner or a 
large burner. Weigh as Mg^V^O^. 

(Per cent. P„0. = 100 X ' ^ ^ — ; ^-'—.) 

* wt. sample 

Note I. — It has been found that precipitating MgNH4P0< in a hot 
solution gives a purer precipitate of this composition. See 
that a crystalline but not a milky precipitate results on the 
addition of ammonia.^ 

8. Total Sulphate. — Acidify the filtrate from the phosphate 
determination with concentrated HCl and add 5 cubic centimeters 
in excess. Boil to expel COg. Add 15 cubic centimeters N 
BaCl.> solution very slowly with constant stirring. Allow to stand 
in a warm place for from 2 to 4 hours. Filter by decantation and 
wash thoroughly with hot water. Ignite and weigh as BaS04. 

^T, ^ c^r^ V. wt. BaSO, X o. -5429 , 

(Per cent. SO, = 100 X , '^^ ^ .) 

wt. sample 

Note I. — The total sulphate determination by the alkaline treatment 
and fusion includes the free sulphuric acid (if any), the 
neutral sulphates, and the sulphate from the oxidation o£ 
sulphur in the protein substance (if no SO3 is lost). With 
sufficient alkali present and with slow heating, no material 
amount of SO3 should be volatilized during heating. 

^ Compare "Analytical Chemistry," Vol. II, by F. P. Treadwell, trans- 
lated by Hall, p. 434 (1919). 



IRON TANNAGE 



83 



Note 2.— I£ the total sulphate alone is to be determined the follow- 
ing method proves to be convenient and satisfactory. 

Weigh a 2-gram air-dried sample in a platinum crucible, 
cover it with 15 cubic centimeters of N/5 Na^COa (approx- 
imately) and allow the sample to be thoroughly soaked in 
the alkali. Evaporate to dryness in an air bath (made by 
setting the crucible into a hole cut in a piece of asbestos 
board so that about one-fifth of the height of the crucible 
projects above the board, and placing the board with the 
crucible on an iron crucible of about 50 cubic centimeters 
capacity). When the sample is thoroughly dried, gently 
char it over a very low flame. Fuse the charred sample 
with 2/2 grams K2CO3, 2^ grams Na^COs, and 2/2 grams 
borax (all chemically pure) after mixing them thoroughly. 
Dissolve out the mass as described above. (FcjOs can be 
determined here in the precipitate.) Acidify the filtrate 
with concentrated HCl, boil to expel CO, and determine 
S04= by BaCla in the usual manner. , 

Remark: Other methods commonly recommended for 
the total sulphate determination are based on the destruc- 
tion of the organic matter by oxidation with (l) fuming 
nitric acid (Stiasny's method), (2) chromic acid (a di- 
chromate and concentrated H.SO^) and (3) sodmm per- 
oxide. With fuming nitric acid and chromic it is very 
difficult to bring the sample into solution even by a pro- 
longed digestion. With the Na.O. fusion, the method 1$ 
more rapid, but it is accompanied with certain disadvan- 
tages. First, that an iron or a nickel crucible, m place 
of the platinum crucible, must be used and this interferes 
with the iron determination if it is to be made here; and 
second that the frothing and spattering of the liquid dur- 
ing fusion is inevitable (unless the sample is first charred, 
in which case some SO3 would be lost). The above method, 
as described, permits the use of the platinum crucible ; gives 
a very quiet fusion yielding a low-fusing and non-viscous 
melt- and loses very httle, if any, sulphur through vola- 
tilization during charring if sufficient alkali (Na.COs) is 
present in the crucible. On the other hand, too much 
alkali will cause disintegration of the sample yielding a 
thick frothing liquid which takes a long time to dry. The 
method is somewhat longer than the peroxide fusion. 
Q Hide Substance (Adapted from Dyer-Kjeldahl Method for 
the Nitrogen Determination). -Weigh a i-gram sample, wrap it 
in a small quantitative filter paper, and introduce it into a dry 
Kjeldahl flask (250 cubic centimeters capacity). Cover the 
sample with 25 cubic centimeters chemically pure 1.84 sulphuric 



84 i,bathe;r chemists association 

acid and place about 0.7 gram mercury or 0.9 gram solid HgSO^ 
in the flask. Clamp the flask at an inclination of about 60° and 
heat very gently until no frothing is seen and the black liquid boils 
quickly. Cool and introduce 15 grams anhydrous NajSO^ and 
three glass beads. Heat until the sample is completely dissolved 
and the color becomes light yellow. Cool completely and add very 
carefully about 150 cubic centimeters freshly distilled water, and 
shake until all is dissolved. Cool in running water. Transfer 
the solution to a 250 cubic centimeter graduated flask and make 
up to the mark. Pipette out 100 cubic centimeters into a 750 cubic 
centimeter R. B. flask, add ^^ gram sodium sulphide crystals, 
NagS.gHaO, dissolved in a little water, and allow to settle. Place 
in the flask three glass beads and three pieces of pumice stone. 
Make up the volume to about 300 cubic centimeters with freshly 
distilled water. Dissolve 10 grams NaOH in about 35 cubic cen- 
timeters water to which is added a small amount of rosolic acid. 
Pour the concentrated NaOH solution into the flask quietly down 
the side without disturbing. Connect the flask with a Hopkins 
distilling head to a Liebig condenser and distill with the delivery 
tubing dipped into the bottom of a 500 cubic centimeter Erlen- 
meyer flask containing 50 cubic centimeters N/io HCl (accurately 
standardized), 50 cubic, centimeters water and 1-2 drops methyl 
orange. Distill for about 45 minutes, when about 150 cubic centi- 
meters of the water will have passed over. Titrate the excess of 
HCl with N/io NaOH. 

(Per cent, hide substance = 

, cc. HCl used up X 0.00786 

100 X ; ^—. X 2.5). 

wt. sample 

Note i. — Introducing the sample into the Kjeldahl flask by wrapping 
it first with a small quantitative filter paper prevents any 
fine particles of the sample from sticking to the upper part 
of the neck. 

Note; 2. — It is necessary not to introduce Na2S04 into the flask until 
the frothing has completely subsided, otherwise, trouble- 
some foaming on subsequent heating will result. With 
the above proportion of H2SO4 and Na2S04 and with the 
catalytic effect of mercury, the sample will be brought into 
a complete solution in 20-30 minutes. 



IRON TANNAGK 85 

Note 3. — Mercury is first precipitated as grayish HgS in an acid 
solution, because it will react with NH3 yielding the mer- 
curic ammonia chloride precipitate Hg.NH2Cl from which 
NH3 cannot be readily liberated. The precipitate of HgS 
does not interfere with the distillation, and quiet boil- 
ing prevails throughout. With NaOH introduced in the 
manner described, the heavier NaOH solution will remain 
at the bottom layer and there is no danger of loss of any 
ammonia before distillation. The presence of an excess 
of NaOH is indicated by the purple color of the rosolic 
acid. 

Note 4. — The presence of much chloride in the sample will cause 
some loss of NH3 during H2SO4 — ^NajSO* digestion, as 
NH4CI is liable to volatilize off with strong heating. 



VITA 

Te-Pang Hou was born in August, 1890, in Foochow, 
China. He obtained his primary education in that locality and 
later in a Middle School in Foochow City. 

In 1908 he entered the Railway Technical College, Shang- 
hai, China, finishing his work in 1910. He was in engineering 
practice for some time with the Tientsin-Pukow Railway. 

He came to the United States in the fall of 1913 and 
joined the Massachusetts Institute of Technology, Boston, 
Mass. There he took up Chemical Engineering, (Course X) 
and was later admitted to the new travelling course (Course 
X-A). He finished his work there in 1917, receiving the 
degree of Bachelor of Science. 

From 1917-1918 he was on postgraduate work in applied 
leather chemistry at Pratt Institute, Brooklyn, N. Y. and re- 
ceived a certificate for the work in June, 1918. From 1918- 
1920 he was in the graduate school of Columbia University, 
New York, N. Y., where he received the degree of Master of 
Arts in June, 1919. 



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